Ask HN: How do services like ZocDoc, OpenTable, etc sign-up providers en masse?

I'm building a service that basically involves lead-generation and customer/vendor matching. My vendors (the lead buyers) are almost always sole proprietors or small businesses. There are thousands in each city.

I don't need to signup each one, but I'm curious how sites like ZocDoc, OpenTable, Groupon, Seamless, etc manage to signup so many vendors before they even launch. My situation is actually simpler than theirs because I don't need to sync with any existing booking platforms like ZocDoc and OpenTable do.

Do they just employ massive sales forces and get boots on the ground? I know Groupon basically does this, but they only need one a day. I need at least a few dozen for my starting city and I need to convince them to pay for my leads.

I don't mind "hustling" and cold-approaching each one at a time, but I'm wondering if there are faster ways, especially for someone bootstrapping and without resources to hire a sales team.

My service is outside the USA, FWIW.

Thanks!