I seem to remember reading that the term Melodian in USA sometimes describes a different instrument to the MELODEON in the UK. You may well find the pump-organ is the correct description, and it would not be what you wanted. The Melodeon in the UK (also sometimes described as a button accordeon) is portable, but not really totally appropriate for Shantys, I think. Concertina is best. You can get both new and secondhand anglo concertinas mail-order from various suppliers, who DO export. Have you tried Hobgoblin?