If we put a number for books and materials not required to be purchased from the school, do we need the receipts for all of the things purchased in order to put a number?

A receipt is not required to enter the amount or transmit your return however, you should retain for your records receipts or supporting documentation should the IRS inquire about the expenses you are claiming.

If you don't have the receipts, you could look up the price/costs of these materials in your area and save for your records.