A week after detailing the 20 hottest digital ad agency prospects, another AdAge piece now estimates that there are only about 10 unspecified shops currently worth the consideration of private equity firms. That’s the assessment of one unnamed banker, who notes that the list of available large independent agencies (or agencies that bring in more than $10 million to 15 million annually) that are amenable to a buyout continues to shrink. And within that group, the banker tells AdAge that “every other broker and PE firm has called all 10 of those guys at least three times.” Attempting to zero in on the number of possible investors out there, AdAge further estimates that there are around 40 PE firms targeting digital ad shops, spurred on by the major payout investors like Hellman & Friedman stand to make for arranging the pending $3.1 billion acquisition of DoubleClick. In pursuing deals with interactive ad shops, some PE firms are fashioning a business model that resembles “a mini-holding company.” One example of this approach is found in General Atlantic’s February purchase of a majority stake in San Francisco digital ad agency AKQA. In seeking to build mini-holding company, AKQA bought search-marketer SearchRev in August. That purchase of a search-marketing company also points toward the direction PE investments will likely take, as more general interactive ad shops like Blast Radius get snapped up by the likes of ad agency holding companies like WPP Group, which have increasingly challenged PE firms in seeking out acquisition targets in the digital ad space. So as the past year’s purchases have tended to focus on general online ad agencies or networks, at least one banker specializing in these deals, Michael Seidler, CEO of Madison Alley Global Ventures, sees the next wave of purchases washing over niche-service providers offering search, e-mail, widgets, desktop and mobile applications and analytics services. There’s possibilities seen at the other end of that spectrum as well, with with investors willing to target offline ad companies that have some degree of digital disciplines within.