These sites deal with the helikon or tuba bass button accordions.
Classical Art Fabricators These folks in the Upper Peninsula of Michigan repair and rebuild accordions and carry a stock of used instruments.
Fontana Button Accordion Club This is a California organization that has, among other things, a page where people list used instruments for sale.
Your Accordion Service This is a part time operation in the Lower Peninsula of Michigan by a computer worker who does accordion repairs, imports new German accordions, and plays in a German band.