Patrick O'Shaughnessy's collections of songs found in Lincolnshire are all out of print, and all worth having. There aren't any songs collected by RVW in the three books mentioned above, though, as "Shimrod" has said; most were recorded and/or noted by Percy Grainger.

Roy Palmer's Bushes and Briars: Folk Songs Collected by Ralph Vaughan Williams is also an excellent book, but certainly not "hard to get". It is in print, and available new, at a good price, from the publishers:

Llanerch Press.