Paying material suppliers directly and getting the waiver from them is the only sure way I know of. And it's the safest route for you.
If you pay the supplier for the materials yourself, there is no need for a waiver, you have the signed receipt. That's proof enough in any court.
If the contractor pays them, then you do need a waiver.
We just finished (in March) building our new home. Some we did, some the contractor did, but everything they did was paid through the bank on a "voucher" system. The voucher was like a check. The supplier got the voucher, for the amount owed at the time (in case it wasn't all materials needed, only partial delivery). In order to get his cash, the supplier (or sub contractor) had to sign the back of the voucher, which was a waiver, and give it to the bank. The bank signed it and handed him a check. That way everyone was protected, and folks got paid every friday for work completed.
Subs like them cause they get a regular paycheck every fri, if the job is longer than one week.
Tell your contractor YOU will pay the materials directly, and give him the rest. Then you will have a receipt (or a waiver too, if you so desire) for materials and a waiver for the rest from the contractor.