1) Open up Scrapebox
2) Harvest proxies
3) Test proxies (decent CPU w/ 100 threads, timeout 30-60 seconds)
4) Keep anonymous ones & ones marked as "Fast" (I think Fast indicates an under 10 second response, not 100% sure though)
I do this every night and it takes me about an hour to test 30,000 proxies. Out of those, about 2,000 work, and about 800 are Google passed. They last into the next morning (I scrape overnight).
Or just buy some. There are a lot of providers on BHW under the Proxies For Sale section who offer lists of 2-3k working, public anonymous proxies updated 2-3x per day @ $20-$30 per month. Even these paid lists are going to be hit by spammers hard though.
Finally, you can just buy private ones, but it's more expensive, and if you get them banned on whatever you're using them for, then you're SOL. The cheapest provider I have found is NewIPNow - if you post on their BHW thread they double the proxies in your account for the same price.