You will only find stainless "tube" in 38mm, you can make a stepped down section to fit your hose by cutting a short section off and then cut about 6-8mm out of the tube longways and re weld it back together and then slip it up the inside of the 38mm tube and weld it on the end to give you a stepped down section for your hose to go on.

Atlas metals sell all the bends,tube,step downs. If you need any help with them let me know

Cheers