Boston, Worcester, Springfield, Cambridge

Plymouth:
New England Shelving - (website)
(new and used)

Boston:
Kent Store Fixtures (website)
(new only )

Wilmington:
Joslin Display (website)
(new only)

West Wareham:
Franklin Fixtures (website)
(new wood fixtures only?)
The densely populated areas in and around Massachusetts will offer plenty of buying opportunities for prospective retailers, though it is far from an ideal location for optimal pricing.

Boston MA is on the coast, which presents two drawbacks when seeking used gondola.
First, it is a tough shipping point from many locations.  Heavy east-coast traffic can for a higher price (per mile), also a coast is a dead end for many routes, such as highway 90.

Being so densly populated in general, the east coast will have very viable options.

New york city is only 200 miles away, and many other metropolitan areas in and around Massachusetts should offer business owners a viable liquidation sale or dealer. There are major dealers in Philly or New Jersey.
Try these nearby states:

New York

Pennsylvania

New Jersey

Ohio