I'm sorry, but could you clarify what you mean?
If you mean tubestock, there are several places that ship, but my team uses a more local supplier,
Discount Steel, which also sells aluminum tube stock, hex stock, and all sorts of other things.
If you mean an extrusion (one of those ones with a funky profile), then you're looking at 80/20 or Bosch extrusions, which can be found with a quick google search. There are probably other manufacturers, but those are the ones I know of.