It all depends on what you're looking for either quantity or quality. Really that's the debate here.
If you want quantity Internet Archive: Wayback Machine might be your friend. However, often you aren't able to find truly unique content, since someone most likely scraped that content before (or when that article was live).
You can go onto elance, oDesk, or even on WickedFire to find some really cheap writers.
You can also do spun content by some kind of spinner program.
Here is what I will do,
Hire someone to create manual spun content, find articles you like, and then hire a manual spinner for super cheap! I mean like $1 buck for an article. It's not the best content, but it pass Copyscape way better than any other option.
I do this all the time, and use this kind of thing for my blog network. It's the only way I've found to get 100% truly unique content without hiring someone to write something new.
To be honest, the rewriters tend to be way better too than original articles at that price.