HP-UX is a proprietary OS that is only sold as a bundle with HP server hardware, or as an upgrade to existing installations. Check online auctions for older hardware, since new systems are far to expensive for personal experimenting (IMO).
And even if it were available for download somewhere, you wouldn't have much fun with it, as it only runs on
PA-RISC and
Itanium processors, both of which are incompatible with anything found in a personal computer.