Bill, Ismael,
You may want to check out www.onlinemetals.com as they have all sorts of metals and some plastics. I order all of my material for them. There is no minimum purchase, they have a wide variety of material, and they do can cut custom lengths.
Shipping runs about $10 from the beginning, so add as much as you can to balance out the shipping.
I've also found that the hardware material is more impure than the stuff from online metals. Especially the brass. The best aluminum to get good results is 6061 T651 heat treated. The aluminum is very dense, and polishes up to a mirror finish. It turns really nicely too. Also, when shoping for brass look for 360 H02. This machines beautifully and machines a fine powder instead of big chips. Online Metals carry both of the aboved grades of aluminum and brass.
Ismael, if you have an aluminum supplier try to get 6061 T6. Other aluminums, 2024, 3003, 5052, dont produce nice finishes. Also, anything less than a T6 heat treat wont have a nice finish. I've never tried 7075 on my lathe yet, so I dont know about it. Though, any good aluminum supplier should carry 6061 T6 as its one of the most common kinds of aluminum.