Check as well with your Principal and what type of budget you have for books. I'm sure for AR books you'd have a bit of a budget to work with. Along with garage sales, etc., watch for sales in bookstores or large depot stores as they often reduce their stock and have good deals. My brother came across one store recently where they had all these wonderful hard cover books for $1.00 each. Hit those thrift stores too.
Check used bookstores as well.
Decide on having a home reading program and write to the book companies such as scholastic and ask them if they would be willing to donate a few books for prizes for your home reading program. The last time I asked they sent me a box of books and I had a few left over to add to my collection.
Talk to any relatives or friends with kids, they often have books piled up around the house, they might be happy to reduce the number by donating some for your classroom too.