Where can I get a version of Pax Imperia for Windows?

Solution 1:

Pax Imperia was never released for Windows; it was a Macintosh exclusive. Its sequel (Pax Imperia: Eminent Domain) was, however, and is usually considered abandonware.

As the original game was for Macintosh OS 7, it is unlikely that it was ever ported to another platform. I'm sorry.

Solution 2:

While you may not be able to get a copy for Windows, if you still have your Mac copy available, it may run in SheepSaver, the Mac OS 9 emulator. Given the age of the game (and thus, the unlikelihood of requiring advanced graphics support), it should run fairly well on modern PC hardware in emulation.

I've used SheepSaver in the past, on prior PCs, and found it pretty reliable and workable. (Unlike, say, Executor.)

If you act quickly, you might even be able to purchase a new copy of the game itself on eBay.