Enterprenuer and founding member of 2 startups - MLSEarthview and MyFeedSpace.
I've done lead gen for branding entities such as yourself.
I would recommend working with multiple lead generation entities, whether that be a firm or an individual, on small scale lead gen projects to test the waters.
Those who do lead gen will be able to hand you a sample of internet mined leads as per your requirements. The lead gen consultant should provide their own lead gen methods and techniques.
Personally, I teach the lead gen technique I use with my clients so they have it for future endeavors. They have other work to do, that's why they outsourced to me in the first place, so this doesn't bother me. If they like my work the first time they will come back.
This is especially true for the three markets you mention that I feel are your best bets - entreprenuers, consultants, and small businesses. I would add to your list internet marketers. So many people out there trying to make money on the 'net.
But remember, there are different ways to segment to a target market on the internet - other ways of defining who and where your target is. You can do this with location and time-based mining techniques.
As far as focus is concerned, I would focus on knowing the type of lead you want - how current you want the lead to be for instance.
If you would like to know more I take calls for questions and I will give you lead gen techniques pertinent to a branding botique such as yours.