Stag Arms ....
If you check on AR-15.com, there are a couple of dealers in the Equipment Exchange selling Stag Arms at pretty reasonable prices. NWTactical and Legal Transfers are the two I have used with very good results. Both are rated high and are AR-15 board supporters.
My LH upper worked perfectly! The only thing to be careful of is the dust cover. It hinges up to avoid blocking the bolt release and sticks up a little higher than the flattop. It lays up against the ARMS base for my Aimpoint so I put the quick release lever on the right side to avoid catching my pink parts on the door.

Once you get used to it being there, it isn't problem.
Good luck.