re: Where to get a cheap iPhone 5s for att?

Depending on how patient you can be, I would expect to pay anywhere from $300-350 on eBay. You may be able to find better deals locally through craigslist, but those will disappear with the quickness.

The size hd you want will affect price as well. The 64gb are usually $25-50 more on average.

Here is a link to an eBay search for iPhone 5S on ATT, used and with a Buy It Now price. Auctions are generally a hair cheaper, but I usually find they aren't worth my time.
iPhone 5S, ATT, used w/ Buy it Now option