Well public proxies are going to die, private proxies would be ideal. It doesn't matter on the link extractor anyway though, because it doesn't use proxies.
You can also find the sitemap url of the site and use the sitemap addon to pull urls.
Also you can scrape the site: operator from google
You can use the full link instead of just the domain.com as well. Google will give results if you use inner page urls, but trimming to root usually yields the best results. Yahoo doesn't support site: and AOL is broken ATM.
Yeah everyone uses public proxies for this, so they die quickly. You an usually pr check a thousand or a few thousand urls on your own IP before it blocked and it will unblock in a matter of hours or a day. Then you can always find new proxy sources, there are countless sources. $30 will get you 10 private proxies and if you set your connecitons to 2 you can practically pr check all day long.