Shearing is probably the cheapest way. The bowing is probably from the material itself and not from the cutting. You could use waterjet, plasma, bandsaw, etc. but I doubt any of them are cheaper than just getting it sheared to size right at the distributor. Then you don't have to pay to have someone else handle it.
If you are a little flexible on dimensions and/or material, you might be able to find extruded aluminum and then just cut it to length. Any way you go, aluminum extrusion and sheet is not known for being terribly flat.