Mushroom vents can be found almost anywhere. There are numerous Threads here about them. Do a Search on "mushroom vent" to find more information.
Amazon has the two piece replacements.
https://www.amazon.com/d/Automotive-...room+roof+vent
Here is a link to a site that sells just the top piece which is all most need. And it is much cheaper.
https://rvandmarine.com/product/mushroom-vent-cap-only/