Mozerella Curd

Where can this be had in the Boston/North Shore area? I tried the major chains, as well as Trader Joe's, a yuppie market in Swampscott.

Are there any obvious substitutions if I can't find it?

I don't know for sure but you should try Formaggio's in Cambridge or Formaggio's in South End Boston. They are the most extensive w/ cheese in the area.

Bread & Circus has a pretty large cheese selection also.

Maybe at Capone Foods in Somerville. They don't have a gigantic cheese selection, but they're unquestionably one of the best Italian groceries in Greater Boston despite their small size -- they may well have what you want, and they'll definitely know where to get it if they don't carry it themselves.