As more chapter books are added, children will be less inclined to want the crates to rotate which may also extend the use of the collection. Depending upon the circumstances of the parents in the class, sometimes parents are willing to donate books since they know it goes for a good cause. Send a letter making them aware of the book shortage. You might want to include titles you would love to see added to the collection to ensure that the criteria for the kinds of books you need will be met by the donations. You might suggest to parents that a lasting gift of a book inscribed with a student's name would be a wonderful gift to the class in honor of their child's birthday or some special occasion.
Garage sales can sometimes be great places to find additions to the class library.
Do keep in mind, though, that students usually choose books that are attractive and appealing. Tell the officers of the PTA about your needs. Give them a list of books and prices and see if they will meet your needs. If your school has a business partner, give them a list of book titles and prices and let them know what you need. Quite often these business partners just don't have a clue how to be helpful, and they would really be grateful to be given something specific.