That is a "Mushroom Attic Vent". You can buy them at Amazon, Walmart, RV Stores and such. There are two pieces to the ones you typically buy. A "base" unit that is screwed to the roof and coated with Dicor to stop any leaks. The second piece (the mushroom) screws onto the "base". You use the new mushroom with the old "base". There is no need to replace the base.
Some tips:
o - Paint the new mushroom top to keep it from being destroyed by the UV light.
o - Someone here (Daryles) had a link to a place where you can buy just the mushroom piece. Here is the link. It is only $5.99 but shipping is extra. Amazon gets $20 for the 2-piece model.
https://rvandmarine.com/product/mushroom-vent-cap-only/
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