One of the best places I have found that sell postcards are Hallmark stores. Not sure where you are located, but do know there is a store in Seymour, and one in Bristol that have a decent collection. If you are driving on I-95, the rest stops do have some - but not a large amount - and many times more for Massachuesttes and New York. Another place to might be the Mystic area. I haven't been there in a while - but as it is a large tourist spot - I am sure there must be some. (if not the stores on the main street than the seaport itself)
Old posters of Cornwall are available in the Cornwall Postcards website as well as other websites pertaining to the sights and landmarks in Cornwall and its neighboring places.
Buying Postcards of PaintingsWhy buy postcards when u can afford to buy online - real original paintings? There are several web sites selling reproductions.
The best place to buy collectible postcards would be in auction sites such as eBay. You can also go to postcard auctions and bid of collectible postcards.
One can buy holiday postcards at stationary stores like Carleton, Hallmark stores, Staples and Grand and Toy. Other location one can get holiday postcards are through Zazzle and Amazon.
The address of the Antiquarian And Landmarks Society is: 255 Main St Fl 4, Hartford, CT 06106-1821
Save the date postcards can be purchased from a variety of online design and print services. Some such services include the websites Postcards, Zazzle, SaveTheDates, and 123Print.
One may find pictures of Canada from various sources such as, an atlas, various magazines, various websites, and postcards. With the picture being of higher quality postcards are a popular medium in which one can have as a keepsake for more memorable landmarks.
One can buy "save the date postcards" in every copyshop. Another possibility is to purchase them online in the different online-shops, which are specialised in printing such cards.
you'd be better off drawing them yourself
A few sites have "real" postards for sale and mailing online. By "real" postcards, this answer is responding to the question for travel postcards, sites from around the world that have actual printed paper postcards from cities and sites around the world. A couple sites that I recommend that you try include: 123Greetings, and Zazzle.
A central location for it is CT beverage in middletown.
You can try searching for interested individuals through forums dedicated to postcards and stamps. Auction sites like ebay.com are also a good way to check if there are people who wants to buy old postcards.