I called part dealers until I found one. You will need photos of it with basic dimensions to share with them because the part number Is not on it. It is best if there is a ruler in the pictures. By about the 100th call I found one.
Mine was actually at a dealer of full size chevy car parts. Because none have numbers on them they get mixed up and confused with the wrong models often. It was in a box of a few dozen impala heat tubes, if not for the photos, the dealer never would have known he had it. It was new NOS GM. I paid $75 last year for a 1968 heat tube. Tough to find but that is the best advice I can give you in locating one.
John