I think you can get ATT Straight Talk micro sims still on ebay at a marked up price. Net10 has ATT micro sims, but for some reason Net10 ATT micro sim stock comes and goes depending on area code. If for some reason your area code isn't carrying Net10 ATT micro sims check back after a few days and it might come back in stock. Otherwise, I think you can get Net10 ATT micro sims on Amazon anytime you want. Don't know about Simple Mobile, but Red Pocket Mobile is another ATT mvno but seems to be for people who need international options. AIO Wireless, a new ATT mvno, is only available in three cities, but I think it will be rolled out nationwide eventually.
I just saw your other post. Keep in mind the ATT speeds are much slower than Tmobile. I've been using Net10 ATT for a month because I didn't get very good Tmobile coverage. I have been very happy with Net10 and the coverage is much better than Tmobile in my area, but data speeds are much slower. I think average 1-2 mbps, but can often go below 1. The highest speed test I had was 5 mbps. Tmobile was much faster, but the trade-off works for me because I'd much rather have a signal and slower data speeds than no signal at all.