I have used flex pipe to connect water heaters, and the flex pipe nut screws right onto a 1/2" diameter galvanized plumbing pipe. I think the i.d. of the tubing itself should be close to 3/4".

I tried to find some on the web for you, but for some reason many sites say "not available online", so I couldn't review specifications. Odd, I don't know why they are available in the stores but not online...

Gas pipe for natural or propane gas comes in different sizes and should work fine too. You might try propane suppliers?

AV8TOR