Good luck searching, that is a rather uncommon size as far as metric taps go. Maybe a specialty tool place, and don't forget you need right and left hand thread versions. I know Marlin had a tool maker cut him a left hand tap for that size when he started making his HD tie rods back in the late '90s, I was at his shop the day that tool came in. I forget how much he paid for that, but you could buy a lot of tie rods and drag links for what that tap cost.

Or try a machine shop that can set up a thread cutting bit and use the thread chasing gears on the lathe to cut the threads inside the tube.