How should I search for packages to install on CentOS 5.5?

***Backstory:*** Recently, it was explained to me that to upgrade any package via terminal on a linux machine, I will need to use the distributions package management system to install or upgrade the package. Since I use CentOS, which is a RHEL variant, the rpm command will need to be executed in terminal to accomplish this (I believe so). Therefore if I need to upgrade or install a package I will first use the wget command to download the package and then the rpm command to install it. The process is clear till here! ***The actual question:*** However, to use the wget command to download the package, I will need a url that points to the package. How should I find this url? My personal research has shown that there are sites like rpm.pbone.net (the only one I know off) to search for these packages. A search for 'firefox' (selecting search for rpms by name) as the keyword has given results for a whole lot of distributions. CentOS 5 is listed on page 3 but the latest version seems to be 3.6.18. Given that version 5.0.2 is available for Fedora (another RHEL variant), where is the latest version of firefox for CentOS? I am unsure which package should I download to upgrade firefox. ***Plea:*** It'll be great if someone can point out how should I go about searching for packages to install on CentOS. Is there an official site for CentOS packages similar to rpm.pbone.net (which I believe is unofficial). I am currently, just for practice, searching for the mozilla firefox and vlc's latest releases.