Why don't you engage your current Business clients/customers to see a) how they would promote your marketplace if they were offered an incentive b) If they would be apt to telling similar businesses that fit the marketplace niche? This way you are not going in guessing but have actual data on your target affiliate and customer.

From that you can pick from a variety of partnership or affiliate platforms to manage the activity and then you (or hire someone with this skill) to recruit affiliates or partners that would be a right fit.

I have more ideas - up for a call if you are:)