There are those out there with far more experience than me who could give you the quick and dirty answer that will help you, but in my experience it seemed to come down to the type of business you're in in. In my case, I'm launching my own site selling ebooks. Finding data on how many ebooks sell in my subject field is like pulling teeth. The only thing I could find were those expensive marketing reports online and they costs an arm and a leg. Sorry, I don't have 2K to spend on a report, esp. if I don't even know if it will give me the answers I need. so I did it the old fashioned way: I found the top few sites on Google in my field that sell ebooks, got a feel for how their sites were set up, how often the site got mentioned when you Google it, etc. That's about the best I can do. I'm also looking for an org that tracks ebook sales. No luck so far. Amazon and Barnes & Noble might release figures, but I don't know of any independent sites that do. So I get where you're coming from.
Ask around to people in your industry. If the ones in your area won;t talk to you (competition) then call someone across the country. That means getting through the gatekeepers, which means being creative or else knowing someone.
Oh yeah---try your local colleges, they should be able to point you in the right direction on this. Hope that helps.
" Difficulties mastered are opportunities won"