Well, the easiest way to find one is to use the lists of proxies.. they have spiders discovering them all the time.. however, about 95% of them are misconfigured and the admin has no idea, so you shouldn't be using them. 5% are public proxies that allow you to use them.
The best way to find one is to either make your own, which provides the best performance, but worst anonymity, or by talking to the network admins. If you are in college or university, there may be a proxy they have available for students that you can use. In fact, they may have it set up in a nice large proxy network that will get you extremely fast results.