I did some searching, since bellows help seems a little leaky. (Sorry) The Button Box seems conveniently close - a mere 140 miles away.

Here's a list of repairers with addresses from the Concertina FAQ. Besides the Button Box, there's one listed in Cambridge, MA - a bit closer than Amherst.

Stagi website is here. I couldn't see any parts for sale, but there's an e-mail address on the page.

Don Nichols may have some advice. (Just wanted to put the link in here, because there's a picture of that monster Wheatstone contrabass.)