I use a mix. Yahoo gives me a lot and occasionally tv.com has them. zap2it.com has the daily ratings. It's really hard to find ratings for all shows and you really have to know how to read thee ratings.
Stations/networks pay a lot of money for the detailed breakdown of ratings, so they don't give that info up easilly. Variety magazine and Broadcasting & Cable magazines are the best places to look for the avg Joe, but they are expensive and not available everywhere.