Where can I find a replacement bathroom fan with this uncommon electrical plug?
You shouldn't be seeking out that connector.
The problem is, you have not removed the fan. You've removed only part of the fan assembly.  The thing that plugs into is more parts of the fan assembly. Remove it too.
That particular fan is designed to break into halves: there's the half that mounts in the ceiling (first) and then half that latches into it once it's installed.  The two halves interconnect using that &quot;proprietary&quot; connector (meaning: it's peculiar to that make/model of fan; fans generally do not use that connector; it's not a standard.)
Just take the rest of the fan out of the ceiling until you are looking at a Romex cable and/or a junction box.