You might want to look at www.ascmetals.com and remember that Aluminium can have both a crown and warp to the sheet. This means that unless you are buying engineering grade that you will most likely have to face the metal or have it faced to bring the surfaces into true parallel. Depending on how large the section of plate you are using this might have no effect, on the other hand the wider and long a plate is that has not been brought to tolerances the more likely the piece will have warp or crowning and that means surfacing/facing needed to be done to bring it into tolerances.

Just a few points that you might want to consider when looking at buying as the cost in material does have a bit of a reason behind it.

Michael