I buy 30" x 20" sheets at the local hardware store and cut them down as needed. Using a regular utility knife and straight edge, I score the metal several times and then flex it back and forth until it breaks. Then I smooth the edges and round the corners with a file, otherwise they dig into the rubber base of my vacuum frame, but that's it. No other preparation required.

I reuse these sheets many times, so their finish isn't that important to me - they don't need perfectly straight edges for instance, as I cut them bigger than the paper size. If the aluminum is to become part of the piece itself, then you may want to consider an alternative method.