People aren't being helpful because you are inquiring about a grey area of the law, and due to people's distrust of the laws and lawmakers in the People's Republic they suspect that you are an undercover apparatchik for the State or an anti-gunner trolling for incriminating information which will somehow be used against gun owners.
Here is your most reasonable answer: If you want to obey what Healy imagines to be the "law", you could post a "want to buy" ad on Armslist, pay an exorbitant ass-reaming profit to the owner of the stripped lower and before purchasing it ask to see "proof" that the lower had been purchased prior to the Healy decree that owning such postban lowers has been illegal since 1994. Then you could build it out and (if you choose to follow the ACTUAL law as written, dutifully register it via the Mass Firearms Portal. You probably won't be arrested. Enjoy your rifle.
or
You could research serial numbers and dates of manufacture and find yourself a true pre-ban lower somewhere on the internet or in a gun shop out of state. If you have simpatico friends in free states (particularly the south and southwest) you could ask them to check their local gun shops for you. That friend might find an "old beater" AR for sale in a hole-in-the-wall gun shop quite cheap, offered for sale by someone who is ignorant of the values and identifiers of pre-94 aR15's and who is selling it at the price of any other beater AR15. Then you arrange to have the rifle shipped to a MA FFL who would be willing to transfer it to you. Ask you preferred FFL before you buy, and show him your research indicating that the rifle's serial number indicated that the date of manufacture is in fact pre-94.
Another avenue--if you are a socially well-regarded and well-connected fellow--is through your social network. There might be someone at your local shooting range/gun club who was bright enough to stockpile them while they could be had cheap and set you up as a personal favor, and sell it to you at their original cost. That would be a friend indeed.
Lastly, you could break the "law" that Healy decreed via press conference and just do whatever the f*** you want, and be willing to pay whatever the penalty was from whoever was willing to enforce it. That would save you say five hundred or a thousand dollars on your build, but might (repeat: MIGHT) cost you ten grand in legal fees after you get a flat tire on the way back from the range and a helpful State Trooper stops to see if you are alright and notices the rifle case in the back of your car and wants to know what is in it.
If you decide to go that route, google "80% AR15 lower receivers" and read up on what is involved. Apparently people are doing that, the foolish lawless bastards, because they feel that they have a Constitutional right to do so, and they refuse to let the impeccably well-intentioned communist maternal instincts of the State discourage them. YMMV.
All the best, and welcome to Northeastshooters--such as it is.