Word of mouth from neighbors and colleagues, mostly. I mostly DIY small things around the house, but when I have a plumbing task I can't or don't have time to do, I've had good luck in calling those plumbers, getting an upfront quote (often, "here's our trip charge and hourly rate, and a toilet repair is going to be less than the one-hour minimum and around $75 in parts")

The reason I do it myself is that $75 in parts is actually $20-30 in parts and I can order parts and then do 45 minutes of plumbing work faster than I could figure out what plumber to call, arrange a time, and then stay home to let them in.