Here are some of my favorite places to find books:
1. Garage Sales: Get there early for the best bargains.
2. Salvation Army/Goodwill: Ask the local salespeople when they usually get the books out on the floor after a weekend. Lots of people dump their extras from garage sales or weekend cleaning at these places.
3. Barnes & Noble clearance tables (and ditto the Barnes & Noble card... saves me money!)
4. Book clubs through school: One of the 2nd grade teachers ALWAYS gives me book order forms, and I almost always order something! Works great for her by earning her points and it's great for me!
5. Another idea: Have a "book shower" for the music room. (Run it by your principal first to make sure it's ok.) Send home a suggestion of books in a letter to the parents. Have them buy a special book in honor of their child's b-day. Print up bookplates on your computer and you can fill in the child's name, date, and reason for donation. You might mention to the parents in the letter that if they have any good, used books they'd like to donate to the music room, you would be more than glad to put them to use.