Anderson Design Group has a few different sets for sale for under $1 per card.
Also, Lantern Press has some fantastic cards. I found a nice selection at the Space Needle Gift Shop in Seattle, WA. Unfortunately, to buy them online, you have to purchase wholesale quantities.
Here is a fun site that sells unusual USA map cards - Hold the Mustard. They are based out of Berkeley, CA and sell maps like "The Pessimistic Map" or "The Lover's Map". Their prices are also pretty good with no minimums. (N.B. I have not seen these cards, so I do not know about the quality of them).
