Just have a local FFL do the transfer for you.
The only gunshop I can think of in your area is The Gun Room on Rt 9 just down the road from Spags in Shewsbury (Worcester). I was at his place not long ago. I really don't know if he will do a transfer for you, as I don't know the people. Go further west on Rt 9 to Belchertown, and there is R&R Sport Shop. I do quite a bit of business with these people, and they treat me well.
I use a local FFL just east of Springfield to do my transfers. If you find the deal, he will do the transfer for a flat fee (usually $25.00).
AAA Guns in West Springfield is a Bushmaster dealer. He doesn't stock lowers, but can get them.
I have been to most of the local gun shows and no one seems to sell just lowers.
There is a place in CT right on Rt 15 Berlin Turnpike, called Hoffmans, that has RRA complete lowers in stock. If you do not live in CT, you just need to furnish a valid drivers license and wait 14 days until you can take the lower (or any other long gun) home. (directions from Worcester: RT 90 west to RT 84 south to (left hand exit off of RT 84) Rt 15 south. Hoffmans is just past Hooters on Rt15 northbound, right next to the Bedding Barn)
Feel free to email me for further information. I could dig up the addresses and phone numbers if you like.
OSA