Do CentOS RPMs work with RedHat?
I am having trouble finding certain Redhat EL5 RPMs but there are a lot of CentOS versions out there. Would they work or are they completely incompatible with RedHat even though they have "el5" in the name?
Yes, CentOS is for all intents and purposes an exact copy of RHEL.
Indeed they do as Martin outlined. If you're having trouble finding packages for Redhat, use RPMForge. They maintain a massive package repository for Redhat, RHEL, Fedora, and CentOS users.