Where can I find this…

I ran a 4" piece of plumbing copper for almost 6 years while my heater control valve was out.

Never had any leaks without the flange.

You could try galvanized pipe, which would look close on the outside, and no one would see how thick it was, but being super thick might restrict the flow a bit. No flange, again.

You might browse through "Yogi's" street rod parts site.
They have a LOT of generic pipe and hose parts.
That's where I found 90* fuel fill elbows for my 66/67 gas tanks for $18 versus the repro hoses for almost $90 each.