Speedy Metals seems to get good reviews over at
www.practicalmachinist.com. However, always try to compare prices.
Another one:
http://www.metalexpress.net/ (Oops - already pointed out by Dave)
I've always found local suppliers for the aluminum we've used in the past. The cost was low enough that we would buy 20 ft. pieces of some commonly used stuff, and go through their cuts and drops to get shorter pieces of larger stock. They didn't mind me walking around their warehouse (OSHA rules? What's OSHA?)
