Here are some links to reputable adoption platforms:
Massachusetts Department of Agriculture Approved Shelter and Rescue Organizations – List of shelters/rescue groups that have registered with the Department of Agriculture
Petfinder – Shelter animals available for adoption
Adopt A Pet – Shelter animals and animals whose owners are looking to rehome them
Get Your Pet – Dogs and cats whose owners need to rehome them
How I Met My Dog – Matching site for shelter dogs and dogs listed by rehomers
Be sure to check out MAC’s Organization Members – many of them have pets available for adoption!