Solaris 10 can be downloaded for free (for private use only).
http://www.sun.com/software/solaris/get.jsp
Sun is using lots of commands that are specific to their Unix.
Anything else, you'd do well learning one of the BSD's that are around, as mentioned above.
I agree with the others in that if you know howto use Linux it will be easy to use a Unix box. Instead, once you know Unix, you'll have to adapt a lot to a Linux OS.
The best match would be using either Red Hat or SuSE, because they are widely spread. If you want to go for the pro and stable stuff, get Debian or Slackware instead.
All these OS (Unix BSD or SystemV), Plan9 or Linux comply to the free ISO POSIX standard:
http://www.opengroup.org/onlinepubs/000095399/toc.htm
There are numerous other realtime OS's which comply with this standard such as LynxOS and VXworks.
Cheers,
LePiaf