Most of these aren't especially close to Western MA, but;

Old #6 Book Depot in Henniker, NH, has the best collection I've seen.

In addition, I've occasionally found some good stuff at Homestead books in Marlboro, NH
Toadstool bookshops has some, the best selection is in their Peterborough store.
Then there is Brattleboro books in Brattleboro, VT which has good stuff off and on, I found a nice copy of Through Another Lens there.

A good general resource for finding used bookshops in NE and elsewhere is