I live in West Springfield. Ye Olde Butcher on North Blvd has them. They are hands down the best dogs out there. Natural casings, of course. Ye Olde is a fantastic butcher shop too!
pekarski's smokehouse in south deerfield has them. and i'm pretty sure both the big y in northampton & greenfield carry them at theor deli counters. other markets may as well, but i've bought them at all those places.