CMMG will build a MA compliant version for you in a heartbeat. They have wonderful customer service.
In addition to Four Seasons, I'd try State Line Gun Shop in Mason, NH (200 yards from the MA border). I'm huge on customer service and Paul fits the bill. They have a huge assortment of new and used MA Compliant rifles from all manufacturers along with stripped lowers, lowers w/stocks, etc.
The other place I'd call (before driving) is Riley's in Hookset, NH. They always have two dozen or more ARs in various configurations from all the major manufacturers and a few of the harder to get companies like Colt, LMT, Sabre, etc.
I'm into ARs and that's where I'd check first. Good luck.