In general you can use pkg.org to locate repositories:
Additionally I usually just google for the package name adding/subtracting bits from it's name depending on which distro I'm looking for.
- look for packages named el5 or el6 for either of these distros at version 5 or 6.
- look for packages named f# where # is a number like 14 for Fedora 14 or 18 for Fedora 18.
This is a good list of the repositories available, most include packages for all the variants (Fedora, CentOS, RHEL).
You can see what repos you do have with this command:
$ yum repolist
Loaded plugins: fastestmirror, priorities, refresh-packagekit
Loading mirror speeds from cached hostfile
* base: mirror.ubiquityservers.com
* epel: mirror.steadfast.net
* extras: mirror.ubiquityservers.com
* updates: ftp.linux.ncsu.edu
73 packages excluded due to repository priority protections
repo id repo name status
base CentOS-6 - Base 6,315+66
*epel Extra Packages for Enterprise Linux 6 - x86_64 9,182
extras CentOS-6 - Extras 6+7
updates CentOS-6 - Updates 916