There are multiple sites that you can search for exploits at. Try security-focus.com bugtraq, securiteam.com, neworder.box.sk, google.com, yahoo.com, etc etc.
Bugtraq is great for finding out about new vulns for various OSes and programs. Sometimes they have exploits along with them, sometimes not. PacketStorm has a lot of exploits, tools, and security papers. You'll find OS specific exploits along with application specific exploits.
Your not going to find an OS vulnerability for linux like you would with windows, most of the time the vulnerability is with the services that are running. Find out what services the machine has running and then cross reference those with the OS version. Most of the time when a vulnerability is released for a certain service a list of OS's is give a long with it. You might want to run a scanner like nessus against it. It will tell you about most vulnerabilities that are open on the machine, although Im its not constantly updated and new ones are released daily, it will give you a good idea of what you can do.