First of all... good luck to find HP-UX free on the Internet... and I am talking about an original one without any modification or malicious code. Second, you can get HP-UX when you buy a new server from HP or have a new version when you need to upgrade. Even if I was to give you a copy, thing I won't do, you would need to get a codeword from HP and pay for it. Look at Solaris instead, they have a free version.
A suggestion, have a look at ebay and search for a HP server that already have HP-UX so you can have the server and the OS at once for cheap.