Finding Out-of-Print Sheet Music Books

I have used this book marketplace site to find sheet music books (particularly out-of-print and/or used): 

http://www.biblio.com

It searches most of the big sellers out there (Abebooks. Alibris, Amazon, etc.,) as well as independent bookstores.  

Example of search for out-of-print book I've got some time ago (Easy Violin Pieces, Hugo Frey):

https://www.biblio.com/search......r=priceasc

This is a good violin duet book, that includes piano accompaniment.  I played the version of Londonderry Air (as Danny Boy) in this book in one of the St Pat Parties in the Party Room:

https://fiddlerman.com/forum/p.....n/#p124457

I'd suggest if you want to buy used, make sure to contact the seller to verify that the sheet music book is complete.  Many sellers don't realize that music in these kinds of books have inserts (e.g., in this book, bound piano music, then removable Violin 1 & Violin 2 parts).