Almost all the aluminum I've ordered in the past 4 years has been from Speedy Metals:
http://stores.ebay.com/The-Speedy-Me...QQftidZ2QQtZkm
6061-T6. The surface finish isn't typically all that great (it's good enough), but you can't beat the price.
There's also Online Metals which a lot of other members here like:
http://www.onlinemetals.com/merchant...p=2&top_cat=60
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz