Newark, Patterson, Trenton, Atlantic City

There is only one (that I know of) used shelving dealer in New Jersey, ARSI Liquidators (website), but they are very active and will always have used shelving, among other used store fixtures.

Check for store fixture liquidations in Newark, Trenton and Atalantic City NJ areas, then branch out to the bordering megacities (NYC, Philly) out of state.

New Jersey can be hit-or-miss when it comes to good deals on cheap gondola shelves.
You should find great opportunities for store closings and liquidations, due to NJ's location; smack in the middle of super-sized retail markets.
All areas, from Jersey City to Camden, should start their gondola shelving search  classifieds.  You are looking for store closings and going-out-of-business sales.  Jersey isn't a great shipping point from the west, which might take some nationwide dealers out of the "low price" category.
Try these nearby states:

New York

Pennsylvania

Maryland

Virginia