Finding the 16" diameter pipe you need around here is hard. Its not a normal material from what I've found. 15" is common, and 18".
You can take 18", cut a slice out of it, and plate it together (some people have even tossed it in the oven for a quick bit to get it to take a new, smaller shape with good results).
Or you can just buy them, shipping just sucks. IIRC the best ones are the UHMW ones (the black stuff). I think it was Staun that was selling them.... I could be wrong (its something like that). I've heard mixed reviews on Dean's steel ones. I know he had issues with his first couple designers... haven't looked into them since.
I have a chunk of 18" I'm going to use... cause right now I'm too cheap to buy the ones that are just done.
________ ROLL BLUNTS