How can I search for blogs + ezines based on niche AND popularity?

[-TW]

Here's my plan...

I have an audio cd info product for a niche.

I want to get it reviewed by legitimate blog owners and ezines in that niche (read: influential). But I want to optimize my efforts by sending the cd only to bloggers and ezine owners who have relatively *large* readerships.

I know how to find blogs + ezines -- but how to I determine what their (actual!) readership is? How do I make my search as easy as possible?

Thanks for any advice on this!

-- TW