Danny Caywood and Clay Smith come to mind. A lot depends on your price range, style preference, and wait time. I could list a dozen or more custom builders but most are booked for 2 years or more. What do you intend to do with it? Gauge preference? Some builders are cranking out fine, plainer guns and others like doing decorated guns with carving, wire inlays, engraving, and so on. You could pay from $1200 for a fine used contemporary smoothbore to $5000 on a new one depending on what you want and who made it. We can offer more specific advice when we have a better idea of what you’re shopping for.