After being involved in construction for a couple of years now, I've realized how much you can haggle if you buy slightly larger quantities of material and plan well in advance.
I have teamed up with my sister, father, and a friend. Through this, we have received much lower prices, the suppliers are much more accommodating, and the leftover material we've been able to exchange among ourselves.
A large part of the cost is transportation, and this costs much less if you can fill an entire truck and unload at two locations.
What we have teamed up for includes the following: basic materials, groundwork, timber, windows, roof tiles, labor, appliances, stoves, chimneys, and paint.
My suggestion to you is if you see a construction project nearby, knock on the door and ask if they are interested in teaming up. Or why not look around this forum and find people nearby.
It is important, however, that in your conversations with the supplier, you point out that you are part of a group that has teamed up to get better prices but that you should still have the invoices addressed to each individual.