Can you make your own in a rolling mill from round?
FYI, Project 24 (pp.154-6) in Alan Revere's "Professional
Goldsmithing" goes into some detail on doing basically this although
in his case it's square tubing that's being made.
The technique he describes is based on using a square core to form
the square tubing and then stretching the core so that the tubing
slides off. With a few adjustments in the process the same idea might
well work for making rectangular tubing, assuming of course that one
has access to rectangle-holed drawplates in the shape and size