soupy1957-
Machinery's Handbook should have it. I have an older revision that has mill tolerances on sheet aluminum and sheet steel and these tolerances depend on sheet size, alloy, production method, and nominal thickness. Be careful when talking about "thickness" of sheet metals. These tend to show free-state dimensional variations (aka bow, twist,...) so the mill tolerances apply when the material is held firmly flat.
If you can't find it there then you can go to the ALUMIMUM ASSOCIATION or to a major aluminum sheet metal producer such as RYERSON.