If eBay doesn't have the size and/or grade you want, onlinemetals.com has a nice selection, but not necessarily cheap. If you have something like a HomeDepot/Menards/Lowe's around, they usually have a limited supply of basic aluminum pieces. It's not the best quality though; it bends easily, but it's easy to work with.
Yea - You could also check metalxpress - but if i remember correctly - onlinemetals has 1/8 sheet but only 12" wide - so you would probably have to cut it to size - and xpress dont have anything that size - only .9 thick i think - good luck in your search...