100% referrals from happy clients, or people I’ve worked with in various contexts over the years. If you do great work and manage clients properly, word gets around. A client with 20 staff - in a year, 2 or 3 of those people will be working for another company. If you’ve left a good impression, over time the network effects take hold. The trick is to survive long enough to reach that point :) the first couple of years were tough.