The best way to get RPM is to install Red Hat Linux. If you don't want to do that, you can still get and use RPM. It can be acquired from ftp.redhat.com.
RPM itself should build on basically any Unix-like system. It has been built and used on Tru64 Unix, AIX, Solaris, SunOS, and basically all flavors of Linux.
To build RPMs from source, you also need everything normally required to build a package, like gcc, make, etc.