Real helpful individuals, eh?
Internet, well no shit.
Guess you have no idea either, huh kiddo?
Heat Riser Tube. It's insulated convoluted tubing that was used for a good many years on late model vehicles. If you get a knowledgeable parts store employee they'll know exactly what it is. IIRC you should be able to find large sizes; the sizes used on cars/trucks were in the 1-7/8" range.
OH, you're not playing dumb ...