http://www.onlinemetalstore.com is located in Minneapolis, MN and has 4"x2"x1/8" if you aren't rigidly tied to the 4"x3" size. Of course, I am biased about them. They have been a very generous sponsor of our team.
Why so big anyway? Most frames I have seen being built are based on 1"x2"x1/8". See
http://www.team221.com as an example.