Look around for muffler shops. I got mine free from the scrap bin at Midas
3" diameter, fairly thick walled steel pipe. Too big to do venetian cutaways, but I don't much like them anyway, and it provides stable heat and better contact for the rest of the curves.
Another option would be to get a thinner walled 2" pipe, and smash it into an oval so you get a flatter surface for the wider bends, and tight radius for cutaways... plus makes it wide enough to get the charcoal starter into it.
You can also turn the wood over and bend it "backward" (concave away from the pipe) to tighten bends beyond the diameter of your pipe. But it's more difficult to control, especially with a full width side versus just a binding strip or whatever.
