Determine your budget for props. You may be able to afford to buy all the props you need, but many school plays have very small budgets.
Make a list of the props that you need. Mark any items that you would like to have but are not necessary. If you have a small budget, you may not be able to buy all the props that you would like and may have to stick only to those that are necessary.
Contact local businesses to see if they will donate any of the items on your list. Since a school play is typically a "not-for-profit," their donations may be tax deductible.
Ask for loans of any items that you will be able to return in good condition. Consider asking the parents of your actors, as well as local theater groups. You may even be able to borrow props from the school hosting the play.
Make props that you cannot afford or borrow. In most cases, you will only need a mock up of the actual item, which will be easier to make. Your stage crew should be able to help make props.