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 news outlets welcoming brand journalism with native advertising opportunities, marketers need to be careful that they aren’t misleading or blurring the lines between "real journalism" and straight advertising.