The RPMs can be found on many web sites. The whole ORBit package is broken in two parts (as for most RPMs) : the ORBit package contains the libraries to use it, and the ORBit-devel package contains the necessary files to develop with ORBit, i.e. the idl compiler and the headers files.
I recommend the RPM repositories like http://www.rpmfind.org to find the RPMS that best suit your distribution. Though ORBit is probably installed if you already have GNOME isntalled, you must download the ORBit-devel to develop with ORBit.
Installation is quite simple : once the packages are downloaded, go:
bash $ rpm -Uvh ORBit* ORBit-devel*