A search engine will be the best database in this circumstance. Your inquiry is specific enough that any firm meeting your criteria should be easily discovered through a combination of carefully selected keywords. Any firm not revealed either (1.) doesn't fit your criteria, or (2.) has no real understanding of online marketing. If they can't do it for themselves, what would make you think they can do it for you? Trust your search engine.
With many companies turning to content to help build their brand, some entrepreneurs and CMOs are looking at the wrong metrics. Here is how content-marketing has evolved and what people should be measuring.