I always got proxies from proxylist websites but I never asked me how these sites discover new proxy. So how?
2 Answers
Looking over ProxyFire it seems like a good option. Although, I prefer using protocols rather than programs. So, looking through the features of ProxyFire, it performs some port scanning which Nmap can supply. Socks proxy servers scanning with Nmap.
Above I described how to discover SOCKS proxy servers by port scanning. Another, option would be looking through forums where users dump lists of SOCKS proxies. See, My-Proxy Forum.
Alternatively, the websites may host the proxies themselves. Either by servers volunteered to them, physical servers owned by them, or using data centre VPSs.
Usually, these sites have admins or people that work for the site. I know a person that works for these kind of sites and he said he uses a proxy scraper like ProxyFire and other ones.