I think the best source is your circle of friends. Has anyone used a pro? Did they like the experience? Why? Ask for specifics. If you can, go and see the finished product or ask for photographs. Is your personality similar to your friends?
If this choice is not a possibility, check social media for candidates or contact NAPO, the National Association of Professional Organizers. Interview at least three organizers and don’t be afraid to switch if you realize you made a mistake. It’s like dating. You don’t have to marry someone because you had dinner once.