Just about every poker site
has freerolls. Try a few of them, and see which ones work the best for you. Some of them have huge fields, but only take 2 or 3 hours to get to the money. Others have much smaller fields and take 5 hours. Going by my own (fairly limited) experience, I'd say that Full Tilt's freerolls have to be among the toughest. They usually take 5-6 hours (to win), the prize pool is only $100, and only the top 27 get paid. If you wanna make money from freerolls, I'd say play anywhere EXCEPT Full Tilt!
(Don't get me wrong - I actually quite like Full Tilt. I just don't like those stingy freerolls.)