I think youd be looking for brushed aluminum sheet not billet. Billets are chunks of metal (like a block/cube/etc). Let your fingers do the walking, have a look in the yellow pages for a metal wholesaler
Some of the better hardware stores have sheet aluminum (and sheet steel, for that matter). In the NorCal bay area, Orchard Supply carries that kind of stuff, as do the larger Ace hardware stores. If you can find a really good hardware store, they should have it.
Look under "steel distributing" in the Yellow Pages if they don't sell what you want, they'll know who does. My father in law is a machinist and gets all his bronze, brass, nylon block, and aluminum from the local "steel" supply house. Nowhere in their ad do they mention they sell all this stuff. I guess you just have to know.
BTW, the kind of sheet aluminum you want can sometimes be bought at scrap prices. The guys I went to consider 2 square feet of sheet to be"scrap". Visit a couple of places if you can, they will vary as to what happens to be lying around at the moment. If they have to custom cut what you want the price can triple that of "scrap".