If you're currently using 1/8" thick aluminum, you're using about the thinnest bar stock you'd be able to find. Thinner than that and you're actually talking sheet product.
There are some very strong aluminum alloys available that would serve well. I'd guess you're currently using 6061? How thin do you want to go? 5052 is a good alloy and is quite strong. It is available in sheet, as is 6061, but I'm not too sure where you'd find it at a good (cheap) price. Usually distributors such as EMJ (Earl M. Jorgensen) have a pretty hefty minimum charge.
