Try M-Machine, they have an extremely wide range of materials and sizes, far more, I would think, than any other supplier to the home workshop. Typically, I estimate, around 400 sizes of steel rectangular bar.
They also quote spec numbers, not just “bright mild steel” as some do, so you know what you are getting!
Click on their link under “Shopping Partners” on the left hand side of the home page for this site.
Harold