I also ask people I meet in business about their preferred networks, but only people I would like as a client.
Work your way from there. The networks want to be found, they want your money after all.
Edit, about starting somewhere:
Some people work to connect businesses. Depending on your location, these can be Chamber of Commerce, Science Parks, tech hubs etc. Start by asking there and get yourself connected with the formal connections first.
I Google my way - City + business network.
I also ask people I meet in business about their preferred networks, but only people I would like as a client.
Work your way from there. The networks want to be found, they want your money after all.
Edit, about starting somewhere: Some people work to connect businesses. Depending on your location, these can be Chamber of Commerce, Science Parks, tech hubs etc. Start by asking there and get yourself connected with the formal connections first.