I just checked them an hour ago and they were out. Wow.
As much as I hate ordering from Sportsmans Guide, I ordered this yesterday:
Full M16 Bolt Assembly - 938923, Replacement Parts at Sportsman's Guide
They are marked RG, which if I'm not mistaken, is R-Guns. I would have preferred a more reputable brand but the specs look GTG.
SMG charges sales tax and has high shipping costs but since I ordered a few more things it made the shipping more reasonable. I also used a coupon code I found online which saved me about $17.
Did it say if the bolt was mil spec or not? Also, make sure the gas key screws are staked properly.
I just checked them an hour ago and they were out. Wow.