I found mine at Target. They are very simple white cards with Thank You in pretty silver lettering on the front and a silver border. They were in packs of 25 for $2.
The most affordable option I offer is the digital design ($18). This gives you printing rights to print as many cards as you want, wherever you like. Printing through Snapfish, Costco, etc is about 10-20 cents a print. You can easily write your note of thanks on the back of the card. All cards are customized, your photo(s), wedding colours, text, font, etc. Feel free to contact me if you have any questions.