Here's a few more. Not all of them are necessarily for sale, but they were too good to pass up.

Galway Street Musician (portrait of Joe Gaynor, accordionist) by Barrie Maguire.

The Peasant Dance by Pieter Bruegel the Elder.

The Blind Piper by Joseph Haverty.

Here's a whole website devoted to images of pipers!

The Banjo Lesson by Henry O. Tanner – a great one.

Several Vermeer paintings with musical themes. (OK, not necessarily folk, but homey.)