How do i find the part name for a rubber seal for a plumbing vent pipe for toilets.

[Nicku]

We’ve got a minor water leak in one toilet, my husband has just cleaned the gutters as we’ve had a lot of rain in Brisbane and found that the rubber seal around the plumbing air vent on the roof has perished.  So far I’ve asked on a chat and also at a store with no success.  With more rain due this week we’re wanting it fixed asap. Help is we’re much appreciated!!