Lean on the contractor for supplying the building as well. They usually have decent deals as they work with the vendors regularly. At the very least, it would let you compare pricing.
Owner supplied materials can be a pain where if something isn't correct, any changes or issues are all up to the owner to remediate with the vendor and you can be stuck in the middle while the contractor waits... Waiting costs $ and can exceed any savings you may have gotten by sourcing the materials yourself.
Food for thought...
Sometimes it may be better to let the contractor handle everything 100% as all of the responsibility is 100% his and you don't risk delays and added costs due to owner supplied materials that are incorrect.
