Does the post office sell individual postcards? Specifically, AZ.?

I was wondering if the post office sells individual post cards? I don't really deal with the actual post office, to often. And if so, are they relatively cheap? I know you can buy them at Walgreen's for a quarter, but they have stuff on them that eats up some of the writing space and I need all the writing space I can get!! And they can't be stamped, they have to be able to be sent through metered mail. I know this could sound like a stupid question, just wondering!!