I had the same trouble in the beginning. But then what I did was found more sources for publix proxies. So now when I scrape public I get 50,000 out of those 50k there are only 15k after removing duplicates. i have one source that provides 10k every scrape.
Now I used to test them all which took forever and I would end up with 200 - 400 proxies. But then I quit testing.
So here is what I do, I scrape the 50k then remove dups. this leaves 15k usually. then after this I just save to proxy list and go. And I found that I could scrape at 500 threads and get plenty of urls.
If you want these lists of proxy sources just pm then you put them into SB and you are gold.
now the drawbacks. just like with all public proxies you have a large failure rate when posting or scanning, or doing anything I found that if I ran it through multiple times I would keep picking up successes. Like if I fast post on 5k then only maybe only get 50 or 150 or something like that. but after removing success and rescanning I get about the same number each pass until no more.
Now I have purchased 25 public proxies from squidproxies. they are amazing. You are not supposed to use them to scrape but to post is awesome! they have amazing success and very fast. If you use them to scrape the net they will get banned I have heard so use public to scrape.
Hope this helps. Peace.