There are various sources and they get their data from different books.
The figures shown on the SBR Live odds (consensus tab) come from 5Dimes.
Assuming you are looking for a free service then thespread dot com is probably the easiest. Their data is a slightly delayed version of that provided by sports insights (which is a paid service).
For NFL percentages I really like to use the Yahoo Pick 'em distribution percentages they have for the spread. To me that is about as "square" as you can get. It was unfortunate they only did the major games for college football, otherwise it would be really useful IMO.
But I will say, I am definitely interested in the whole reverse line movement theory. Especially when it can just be one factor in your decision making process. I ended up building a while iPhone app around the idea.