There is no single best place. You can spend a lot of time chasing free proxy lists and hoping to be spoon-fed working "free" proxies...
or you can learn how to leech from the right sources and/or spend a bit of money on a primo source that offers frequently refreshed proxies.
Think about time = money, factor in how much of your time (machine time, human time, or the combination thereof) you spend hunting the elusive magical free proxies (that actually work - don't leak your real IP, and have low latency), and then consider if there are reasonably-priced, quality providers who would be a relatively small investment to save you a bucket of time.