My experience is that the melodeon - as we know it in England, i.e. a diatonic box with two rows a fifth apart - is pretty rare in the US and Canada. The players I've come across are usually expats and/or playing for morris dance teams.

The Quebecois use one-row, ten-button accordions a bit like the cajun ones but much lighter in both weight and sound. They also have some three-rows up there. Tex-Mex players usually use three-row diatonics.

There are lots of piano accordions in the polka scene, which I don't know a lot about, but it seems to be located mostly in the Northern Midwest. Quite a few Klezmer bands use them as well, also some contra dance bands.

There are more anglo concertinas than melodeons around, generally used for Irish music (and guys like Billy McComisky use chromatic button accordions too) or for morris dance.

The Button Box in Massachusetts specialises in concertinas but also sells some melodeons, so they might have an idea about melodeon sales.