The FFR aluminum is 0.040" 6061-T6. 6061 is the alloy, T6 the hardness as JasonD says, graded 0 to 6, 0 being soft , 6 the hardest. 6061T6 is quite strong, but relatively hard to bend--the 0.040 stuff is thin enough to bend OK "once" in a bending brake, but if you bend it back again it will usually fracture and break. Thicker 6061T6 is quite strong, but is difficult to form or bend.
3003-T3 is commonly used on the less-stressed parts of racecars, and is more easily formed by hammering, rolling, etc, into tanks, scoops, etc., and is still quite strong enough for most purposes.
My battery box is 3003-T3 (16 gauge, actually, a bit thicker than necessary because I had a piece laying around).
Best prices are usually at a big metal supply house (Industrial Metal Supply in SoCal), where they usually have scraps of all sizes and shapes, sold cheap by the pound usually. The alloy and thickness are usually printed on the panels in ink, just as on the FFR panels. Where appearance is a big issue, you can even buy it with plastic film adherent to the surface to protect the aluminum, to be peeled off after forming operations. Local sheetmetal shops, heating and A/C shops are also sources.