Right now, your are going to way overpay for on if you can even find one. I'd just periodically look at Palmetto State Armory, Spikes Tactical, Bravo Company, ect's sites. A typical lower goes for $80-$120. Right now, they are going for $200-$300. Just keep an eye on sites like the ones listed. There are hundreds of companies to choose from. Prices will come down, and availability will come back.
With the AR10 platform you need to know exactly want you want to build. RRA,Armalite,and DPMS are the main mfg's,and each one is different.
So if you buy one brand of lower,you'll have to find out what parts are compatible with it. They do not interchange like an AR15.
CMMG,and Mega also produce lowers,and I think they do both DPMS and Armalite styles.