Look like I got the bug to build me a 308 dmr. I'm left handed and since I'll be shooting it suppressed,  I'd like to do a left handed build. I know 308 ars arnt as cut and dry and the 556 ones and theres compatibility nuances. What lowers would be compatible with the stag upper?

Just buy a Ruger SFAR, save some money and have a lifetime warranty. You probably want a 308, but check out a 6.5 Creedmoor.

They make a southpaw version, now?

Stag uses the KAC/Armalite slant cut for their Stag-10's, and DPMS BCG, barrel threads, and tall rail height for the uppers. It's a mish-mash of different standards.

While sort-of compatible with other slant-cut lowers, if you are set on Stag for the LH thing you should really stick with a Stag lower. LMT will work the best, but you will have to open up a pivot/takedown pin hole in the upper a bit to make it fit.