Author here. In your case, probably through the network of your former clients and the networks of your candidate pool. It's tough to do without some cold calling, but when recruiters cold call companies they are saying 'I have a candidate, will you pay me to hire him/her?' whereas an agent would be saying 'I have a candidate that is interested in working for you, and it is free to hire him/her' (or close to free). It's a big difference - you are offering the same service as recruiters offered to companies, but you're giving it to them for free.

If you are making cold calls to tech companies starved for talent, telling them you can solve their problem for free (that they have been paying a LOT for historically), the cold calls will get pretty warm pretty quickly.