CMMG doesn't make them anymore, but others do/did. The design rights have been passed around a few times... I want to say Anubis Arms makes the lowers now. This kind of lower has some similarities to the AR w/ AK magwell builds in that you need to hog out the upper a bit for the magazine to fit properly. I'll also note that I've found mag compatibility with the lower to be hit and miss - some of them fit great, others you really have to jam in for them to lock in.They're not even making those anymore. WTF.
TBH, SR-25 magazines are probably more sensible, especially considering no LRBHO with G3 mags, but the thought of yet another kind of magazine to manage really was not appealing.