NET10 (straight talk by another name). Works great on iPhone 5. They have a package of 3 SIMs for about $15: one or two for AT&T and one for Tmobile. You can then cut them down to iPhone 5 size using sim templates easily found via google searches. We did this. It wasn't that hard and the worst-case risk was $15.
Yes, you can also buy remaining stock of ST sims from Amazon, eBay, etc. They are still around. But you're paying a premium for those, which, in some cases, might be larger-sized SIMs cut down by someone else and re-packaged for sale. If you can cut straight lines with scissors and follow directions on the SIM-cutting templates online, you can just make your own sim card for iPhone 5.