Hi Bob, I don't know of a source for 1mm thick tubing. If you don't
come up with a source, could you use this as a substitute? Hoover
and Strong's thick wall tubing has walls of .5mm, so a 3.5mm tube
will have an inside diameter of 2.5mm. If you inserted a 2.5mm tube
(also with .5mm walls) in the 3.5mm tube (and solder it or not,
depending on what you're doing), you'd have a 3.5mm outside
diameter, 1mm thick walls and 1.5mm inside diameter. Good luck!