Hercules and the turnkey MVS system are probably your best route. Google hercules emulator for details and download information. But be aware Hercules is just the hardware emulation -- you'll need to install a version of MVS to be able to use JCL and so forth. Under IBM license agreements, you generally cannot run z/OS or OS/390 on Hercules in any legal fashion.
Your only other choice, as far as I am aware, is to become an IBM Partner developer in which case IBM will sell you a PC with z/OS installed for a mere $20,000 or so.