I spent a lot of time scouring "Ask HN" and "Show HN" posts to find people who were running revenue-generating businesses. I think I compiled a list of over 140 people. I was able to track down about half of their email addresses and Twitter accounts. From there, I got in contact with everyone I could, and told them about the site. If they wanted to share and were comfortable revealing revenue (lots of people weren't), I sent them a Wufoo form I made. Then I edited their answers into the format you see on the site. There are still 20+ people who were interested in being included but haven't gotten me their info yet.

EDIT: A few people have submitted their businesses since I posted this Show HN, too.