The ones you find on Google will be blocked. Understand that the web filter guys have Bots that surf the net and scan sites. It finds the proxies, and bans them into their universal giganitic overgrown blacklist. Websense,lightspeed,fortiguard etc all do this. You will need to get yourself on a proxy email letter to get fresh ones emailed to you. http://groups.google.com/group/proxlab
It usualy only takes up to 7 days for websense and the other guys to find new proxies and ban them.
So if you find one that works, next week it probably won't.