This information, sorted by western MA county, is accessed from the MA DPH website at: https://www.mass.gov/info-details/about-covid-19-testing#where-can-i-get-a-test?-. From that site, you can also download a statewide listing: MA COVID-19 Testing Sites PDF | DOC
The posted document is current as of January 11th, and highlights Stop the Spread sites throughout western MA.
Stop the Spread testing sites are available in communities across Massachusetts that have continued to see a higher number of residents testing positive for COVID-19. Residents of higher-risk communities are encouraged to get tested, even with no COVID-19 symptoms. Stop the Spread sites are free for all Massachusetts residents. A Franklin County Stop the Spread site will be established in the coming weeks. Learn more at www.mass.gov/StopTheSpread.