A lot of the things said in this thread are quite accurate as answers to the question "where can you find postcards", but some of it also depends on what kind of cards you want, the types of cards offered do tend to change over time and there may be certain cards you have seen in the past that you won't see nowadays, also since cards with special elements that people are willing to pay more for have a higher profit margin it seems there have been a lot of those in recent years. Still here's an over view as to some locations and general information about buying postcards on WDW, First I'll go over locations, in the second part I'll talk about types of cards. It also gives me an idea for a photo essay next trip "where to buy postcards at Walt Disney World"
First of all if you are staying on-site there will almost always be some in you hotels gift shop, if you don't see them ask because sometimes they can be hidden away, also if you are staying at a deluxe or moderate resort there is a greater chance that they will have a card that features your resort hotel, but, sometimes it won't be with the other postcards it will be with the resort specific merchandise for that hotel, once again if you are hoping to find a card like this and you don't see one in your resorts gift shop it can't hurt to ask. At the parks there are a couples of places you will find cards, first off, almost all of the parks have a gift shop "outside the gates" the one for the Magic Kingdom is actually at the TTC, in the building to the right of the ticket booths as you get off the trams (as if you are heading towards the TTC bus area if I have my memories in order) the one for Epcot is off to the right of the entrance gates in an enclosed building, but was rumored to possibly be closing at one point, so I'm not sure if it's still there, the one for the studios is quite visible as it is a more Indoor/outdoor location off just past the leftmost turnstile as you are headed towards the entrance, the one for Animal Kingdom is in a Stand alone building outside the gates (I think it may even be before you get to bag check, but I'm not certain). At the Magic Kingdom just after you enter the gates there are a pair of souvenir stands off to each side before you even go through the entry tunnels and these have postcards too. All four parks have a large gift shop in the front half of the park, Emporium at Magic Kingdom, MouseGear at Epcot, Island Mercantile at Animal Kingdom, and Mickeys' of Hollywood at the Studios, though at the Studios for some reason I also remember getting cards across the street (celebrity 5 and 10 I think).
These locations usually have the best selections in each of these parks, but they aren't the only locations that have cards! far from it. There are also many other locations in the parks that have postcards but often the types at those other locations will be much more limited, one exception to this is the gift shop back near the Toontown Country Fair Hall of Fame, I have seen a larger and wider variety of cards there than in most other locations. In most of the parks there is a location where you can get "art of Disney" series postcards, these are a bit more pricey (3.00 or perhaps 3.50 now), but have some beautiful artwork of the characters and icons of the parks, in the Magic Kingdom these are in the new art of Art of Disney Store located in the Main Street Cinema, at Epcot the Art of Disney Store is located just past the camera shop (and before the lockers) to the right of Spaceship earth as you enter the park. In the studios the shop that has art of Disney postcards is at the end of the Animation tour, at the far side of the Animation Courtyard,but you don't have to go on the tour to visit the gift shop it has an entry off the animation courtyard (but don't confuse this shop with the souvenir stand on the nearby building that houses Voyage of the Little Mermaid. Additionally almost all of the parks have some postcards specific to attractions that are in small gift shops at the exits of the attractions, in past years I have seen pirates of the Caribbean postcards in the shop at that rides exit, and Splash Mountain ones at the exit to that attraction, but not on my last few trips, so once it again it never hurts to ask, but try not to be too disappointed if you favorite attraction does not have a card dedicated to it, other attractions on this "Attraction Specific" list I have seen in the past are Stitches Great Escape (buy again not recently), Tower of Terror (this has been more consistent than most attractions), Mission Space, Dinosaur, Mickey's Philharmagic (only back when it was new though), the cart neat the exit to the Haunted Mansion, The Cart across from the Country Bears (That was many years ago), Star Tours has been fairly consistent, though often more of the cards there are Star Wars movie cards rather than related to the attraction, Rock N' Roller Coaster, the cart across from Who Wants to be a millionaire (obviously a while back!), additionally in the past the countries in World Showcase all generally had a card that was available somewhere in each of those countries, nowadays, instead of having cards in each individual country there tend to be a few World Showcase related cards in the "Disney Traders" and "Ports of Entry" gift shops that are near the Friendship docks at the Future World End of World Showcase Lagoon, Another park that has seemed to focus more on having one shop in each land that has cards rather than a shop at the exit to each attraction is Disney's Animal Kingdom, Chester and Hester's Dinosoar Treasures in Dinoland, Mandala gifts in Asia (surprisingly good selection for its small size), and Mombasa Marketplace in Africa tend to have selections of cards that focus on their park and even there specific "land" for some of them, there is also a "land specific" shop in Tomorrowland I THINK that it's Merchant of Venus rather than Mickey's star traders that has the better postcard selection if I recall correctly. Additionally the water parks, and Disney quest have had postcards generally over the years, though I have not spent enough time at those parks to know where the best places to ask for them are, but of course it never hurts to ask. Downtown Disney has two good locations for postcards, one is in the World of Disney store, the other is the Art of Disney Store, there have been times in the past when in addition to the "Art Of Disney" cards I mentioned earlier they have had some of the "regular" postcards sold in the other gift shops.
Now on to the types of cards while the least expensive types of postcards, single photo views of the parks, have become less common in recent years, these have been replaced to some extent by foldout folders or folios of cards, that while they cost more up front are actually cheaper when you break them down to a "per card" cost, additionally a lot of the reason Disney has been able to fill more of the locations they stock with postcards with higher priced cards is that they have come out with some beautiful and fantastic looking cards, there are shiny foil cards, lenticular cards that feature a look that gives a card sometimes much greater depth, sometimes a sense of movement, and sometimes a little of both. They also sell more larger sized cards as compared to the normal 4 x 6 or continental sized cards, there are 5 x7 cards, and there are the oddly shaped die-cut postcards and many of these cards too are lenticualar or foil cards as well. Then there are the "Art of Disney cards" I have mentioned a couple of times now these are 5 x 7 and are embossed to give them the look of matted artwork, I think you really could just pop a lot of these in a frame as is. The past few years there have also been special "boxed sets" of postcards one popping up every year or two, often when I go to WDW at Mousefest new merchandise has just been rolled out in the past few months, and I've even gotten the "next years" card with the year on it at a couple of mousefests, where the cards had either been "just put out" or appeared during my trip, and since I have NOT been at WDW since last December, and have not been tracking down every possible lead on the Internet for new releases this year, I'm expecting my next trip, for Mousefest to be a very good trip for postcards indeed! Though I did just learn about one new release, and this one has the best price of all, it's FREE! If you are passing by a DVC sales kiosk ask them if they have any sample art of the new Bay Lake Tower you can take with you, they will probably give you a postcard!