A mix of a lot of things is going to be the only recipe for success. You can't rely on just one source. Buy leads, cold call, network like an animal, and bit of good luck will be having you binding policies in no time!
After that, internet leads are the fastest way to get people on the phone unless you like scrubbing phone numbers & X-dating.
Mailers work too. What I have found in talking to agents all over the country is that different methods work in different areas. I know agents in smaller communities that dominate Bill Boards & Radio. That would be too expensive in Los Angeles! So know your market/community. Try things out.
I had a very successful agent I worked with in KY that was in a small town. He had huge success with walking neighborhoods & leaving door hangers with info on his agency & an ear of corn from his parents' farm in the bag!
But for many agents, internet leads work well. If you are interested in finding out more, give me a call or email me.
Most of the successful agents I speak with are doing internet leads, email blasts, call blasts, and social media promotions. A lot of money gets wasted on internet leads though, most of them are garbage.