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.
The main requirement to run RPM is cpio 2.4.2 or greater. While this system is intended for use with Linux, it may very well be portable to other Unix systems. It has, in fact, been compiled on SunOS, Solaris, AIX, Irix, AmigaOS, and others. Be warned, the binary packages generated on a different type of Unix system will not be compatible.
Those are the minimal requirements to install RPMs. To build RPMs from
source, you also need everything normally required to build a package,
like gcc, make, etc.