You can't really rely on one or two proxies, it's always good to have a list of those proxies at least. Proxies, sad to say has a very short life span either it was put out from business or was currently banned by a system administrator in a school, office or a establishment.

I usually go to ProxyInter.Net for their latest up to date list of anonymous proxy servers on the internet. The proxies on their list is guaranteed to be alive and still working because they check and update it regularly, so you are assured you have a working proxy list at your disposal and use it whenever you want.