Would like to know if a repair collar is available for a vent pipe roof flashing?

My vent pipe flashings are leaking due to cracking of the rubber donut which slips over the pvc vent pipe. The metal flashings are fine, only a couple years old, but the rubber is badly deteriorated. A web search mentioned that a repair collar is made to slip over the vent pipe so that you do not have to tear up the shingles to replace the entire metal unit. I have checked with plumbing supply houses but no one has heard of it. Does anyone know the manufacturers name of this repair collar or where I could buy it.