CMMG, ASC AR308 magazines will fit, as will DPMS 19rnd 308, and Magpul LR 308 20 magazines.
Absolutely rare as chickens' teeth right now. Over on the other forum, there are lots for sale at 50-70 EACH. SO, I will sit and wait a bit. It might take a couple months, but I am not paying $60 for a $25 magazine.
You should look at all the forums for sale sections, and if you make it to a fun show, it might be possible to get a bulk buy for maybe 35 ea. if you purchase 4..... if the guy is having a slow day.