Back in the old days (1950s), we used to use cycle 3-speed cable, which was a Bowden cable inner of about .050" diameter. My local bike (pedal cycle!) shop sells lengths of 1mm (approx. .040") stainless steel Bowden cable, which is a bit pricey but ideal for the leadouts in a large stunter. Cycling's a big thing in Britain these days, everybody being exercise-crazy, so we have specialist bike shops all over the place - it's probably the same everywhere.