The first source is the homeowners you have already helped. They know your work, and they have already let you in the house. A few useful notes a year, a reminder before the water heater gets old, a postcard before the first freeze, and yours is the name on the fridge when the next thing breaks. They are also the people neighbors ask when their own basement takes on water.
The second source is the businesses that get asked "Who's a good plumber?" every week: realtors, home inspectors, property managers, restoration companies and remodelers. None of them compete with you, and each one looks good when the plumber they recommend shows up fast and does it right. A referral partner program keeps you in front of them with a regular, useful word and keeps track of who sends you what. It is marketing and follow-up, done on purpose.
Robert grew his own real estate business in Reston, Virginia, this way. He started a monthly newsletter for his past clients and the neighborhoods he wanted to work in, by email and in print, and shared its pages with the companies his clients would need sooner or later: lenders, a home inspector, and HVAC, plumbing, electrical and handyman companies. Before the newsletter, about 10% of his business came from repeat clients and referrals. After it, that share grew to a steady 50 to 60%. Those are his numbers from his own business, so please don't read them as a promise about yours. What matters is the order: the newsletter showed up every month, and the work followed.
Because every returning customer and every referral is tracked back to where it came from, you can see which marketing brings in paying jobs, and none of it goes to waste.