It sounds like you're looking for referrals to specific Digital Ad companies that meet your criteria, and might be potential customers. I don't have knowledge of this space, so I can't help you there, specifically.
But as for the API requirement: can you explain what you mean by a "strong API"? Will your systems be sending information to these companies through their APIs? Or retrieving information, or both?
Wondering if you have considered another option: define an API that your target customers should implement, give them some supporting code, documentation and other artifacts that will assist your customers in implementing this API, and connecting it to their back-end systems. Would this be a good alternative or complementary approach?