In a previous house I bought aluminum fins that wrapped around the gas furnace vent pipe to reclaim waste heat. I would like to buy more but have not been able to find them. Are they available?
You are having a problem because most of the stoves and heating appliances built in the past 10 years are much more efficient and many no longer even require a metal flue pipe. Many use PVC plastic vent pipes because the exhaust temperature is so low, and therefore not much “free” heat to reclaim with the fins you are looking for.
The following link is for a really good supplier of “old time” stoves and other hard to find non-powered household equipment. What you want may not be on their web site, but call as they may have this item or something that will do the same thing.