I get to work in several first grade classrooms as a literacy coach, and will share with you a couple of the ways I've seen this work.
One way--the class has 4 different colors of tubs for their book boxes. Each color group book-shops on a different day. (For example red on Monday, yellow on Tuesday, etc). On their shopping day, the kids in that group get to shop during one round for 5 books from the classroom room library to add to their books from the teacher. (They return as many as they choose.)
Another way--Friday is "shopping day" in this classroom. During each of 3 rounds, a third of the class shops for books to add to their book boxes. It's a bit noisier that the first example, but works for this teacher and class :).