1. The students themselves
First of all, you don't always have to provide the books. Some students will prefer to buy their own, and they like to trade with each other once workshop gets rolling. You can also ask them to donate any books they have at home that they don't want to keep any longer.
I send a letter to parents at the beginning of the semester, explaining to them how reader's workshop will work. I let them know that students will be selecting their own books and that they should ask them about what they are reading at home. (This covers you when it comes to students selecting books that parents may not be happy with; by asking parents to communicate with their children about their reading choices, you put the responsibility in their hands). This letter is also an opportunity to ask for donations of any used books they might like to get rid of. Some will also choose to donate some new titles too!
3. Your principal or department head
Ask if there is any money for you to buy some books. Most departments have some money for teachers to spend on classroom materials, and even if it's a small amount, you can add one or two new titles each year.
4. Ask and ye shall receive
Last year I put out a call on my personal Facebook page. I told my friends that I was looking for used books for my classroom library, and I got several tubs of great reads for my students. I even had a former student send me two boxes from Amazon, full of all the latest and greatest YA books! What a nice surprise that was.
5. Used bookstores
Whether you are spending money from your principal or your own, always buy used. You will get twice as many books than you will if you buy new.