Wm. Marshal's collections are available as reprints, as are the 6 books of Gow's Strathspey Reels and 4 books of the Complete Repository. There are also some 19th century collections available as reprints (Skye collection, Athole collection). I have one of Scott Skinner's, 'The Harp and Claymore', as a reprint, but don't know if it's still available.

Try finding 'House of Musical Traditions' or 'Fiddler's Crossing' on the web. These handle reprints of British isles music in the U. S. [All mine came from HMT]. Philippe Varlet, a frequent contributer to IRTRAD-L, is in charge of books at HMT. I lost my web address for 'Fiddler's Crossing' when my computer crashed. The one in charge of it, Jan something, ocassionally contributes to the SCOTS-L list.

I don't know of a source that one can order from in the British Isles.