I have always had friends in the industry, but if I was looking for a new machinist I would be asking around for feedback and seeing if I could check out their work on other people's engines. If the quality of their work checks out, I would suss out what kind of turn around time most people had. Last thing I would be looking at their shop and having a tour and a talk with them.
I know many people who based the machinist they went with based on price, rather than quality. I'm not one of them!