I shop everywhere for books. Mostly second-hand stuff. My favorite store is the Book Bin in Salem Oregon -it has old, and new books. I found some Henty books from 1908, a book called Airy Fairy Lilian from 1889ish, and lots of other books, music included. The Book Bin has tons of shelves, and multiple floors, but books are still piled up in neat three-five foot stacks on the floor. Everywhere. And there are boxes all over the place too. I found some really cheep comics from the 80's once.
Anyway, that's my top favorite place to go, and I think they sell online. Hope that helped you in your quest! :)
Found a used book store chain in North Carolina this weekend called "Edward McKay Used Books." Went to the one in Fayetteville, NC. It's very neat and organized, easy to find stuff. Not a really big music section, as least for what I was looking for, but a great store none the less. I picked up a Mendelssohn collection, a collection of viola solos and a book about impressions of Beethoven, all for 6 bucks. I love used book stores. Ya never know what you're gonna find.