I've seen a couple of trombones going cheap locally and was quite tempted. Unfortunately they were tenors, and a bass would make more sense in traditional music. (Ideally a bass in G to play in the usual folky keys - there should still be a supply of them out there).

Something that plays a good solid melodic bass line is much more of an asset to a session than one of those watchamacallits with six strings and a drawer-sized soundbox that sounds like a right-arm amputee trying to play a piano filled with porridge.