Basically, you want all of your loading to be in either compression or tension. If you have a tube that is welded (or bolted) to the frame at your control arms at a 45 degree angle to the compression/tension forces from the control arms, half of the forces will be put into the tubing in bending and half would be in compression/tension. Since tubes are stronger in compression/tension than in bending, it is preferred to have the tubes acting in the same direction as those forces. Perpendicular might not have been the right way to put this, but, in most cases, you want to tubes running lateral between the right and left side control arm mounts. The triangulation tubing doesn't need to be real beefy because those tubes won't see that much loads. Think of a 4-point strut tower bar and the size of the main bar compared to the firewall support bars.

Aircraft Spruce is where my team buys most of their metal, but that's in Cali and shipping charges for small amounts of tubing probably outweigh the amount of tubing that you're looking for. I really don't know a good place locally.

For tubing, I would just run round carbon steel. Depending on how stiff you want to make it, 1"x.065 to 1"x.095 is probably what you're looking for. For the traingulation, 1"x.035 will work.

-Joel