Check out www.diyTower.com
for some great technical links. It will be easiest to get Schedule 40 Aluminim Pipe and any local supplier of alumimum pipe can set you up.
Depending on if you use stainless or aluminum, the wall thickness will be between .060 and .188. The stainless will be thinner but heavier than the equivilent strength of 6063-T54 or 6061-T6 aluminim. Post pic's of your new tower!