First, of course, you should always check with your local Yamaha dealer. Please understand, though, that Yamaha does not "certify" piano technicians. We offer training to technicians, so we know many of them quite well, and we do have good business relations with them, but we do not certify them.
The best way to find one is to start at the following website: www.ptg.org. From that you can get a list of Registered Piano Technicians in your area. I would recommend taking that list, and checking with local universities, churches, recording studios, and others who would use the services of a professional piano technician. There will usually be a name or two that will keep coming up as the "go to" person in the area. Keep in mind that this is a person with whom you will want to do business for a long time, so you want someone who is not only technically qualified, but also works well as a service business person, being on time, professional, etc. Once you find that person, keep in mind that they may book well in advance, and may charge a fee for a missed appointment or late cancellation. To keep the best person for your piano, you will want to be respectful of their professional status.