So many choices
. Do you already have "leveled books" for your reading time? If so, I'd look for different books for your classroom library. Books that are of high-interest to the kids, predictable books, author collections.
As far as "how to buy", look for deals that various publishers put together. Sometimes "WilBooks" has great collections for a small price per book. Their non-fiction books are really great. Scholastic also puts together classroom sets for various grades also at reasonable prices. Once school starts, using book clubs (like Scholastic and Trumpet) not only allow kids to order books, but the teacher gets lots of points to "spend" on books.
Look on teacher websites, such as "Teacher Swaps" in your area--some retiring teachers, or those leaving the profession, often have collections they are willing to sell cheaply.
Just some ideas--I'm sure others can their more ideas.