Two possibilities come to mind. First, do you have any credit cards? Charge your materials to one and be sure to pay it promptly. Returns are simply credited to your card. Many businesses are going that route anyway. Seems maintaining accounts is too much trouble.
If a credit card is not an option, ask your supply house if you could set up an account and prepay. You would give them an amount like maybe equal to what you would purchase in a month. You would draw against that "credit" and as long as you had a positive balance you could continue to purchase.
-Hal
