Good idea! I feel I've been lucky as I have known of custom builders since my racing days more than 40 years ago. One was a member of my club and built my first custom which I still love and ride. In the many years since, two builders (and soon a third) have (will) worked on it. That third has already built me two bikes, 4 stems, two seatposts and repaired two others.
The builders (in the order they are mentioned) - Peter Mooney, Ed Litton, Mark Nobilette and Dave Levy/Ti Cycles. Oh, and another builder who built me a stem years ago who's name escapes me. All of these builders were local to me. (I've moved around a lot.)
Ben