I have a sign shop about two miles from me. I get all my thin aluminum there. They have your sought after Gloss black in any size and thickness you want. I always get their seconds at a discounted price, it may have a bent corner or a small scratch on it at most.
The one close to me is the main store. they have another store, I believe it's in Tuscaloosa. I can't remeber the name , but I think it's any thing signs, or signs plus. Any way my $.02