Im in the process of fabricating an intake manifold and am looking for aluminum suppliers, i will need a thick gauge 4-4.5" dia. pipe. And aluminum stock to make the connector to the runners and the TB mounting plate.
Similar to this setup:

 

Search for Alro metal suppliers in superpages.com
Or you can head down to your local highschool and hacksaw their flag pole.
Looks like its 1/4" thick wall on that round tubing.