I would start right here at this website. Click on the ‘Home’ tab and go to ‘Certification/Training’ where you will find ‘Pilates’ as one of the options. You can check them all out and decide which one will best suit your needs.
The Y does offer a pilates certification of its own, but it is not widely available. I think only a half dozen or so of them are offered around the country this year. I am surprised that none of your Y teachers are certified. My experience in teaching sometimes in a Y is that some instructors carry national certifications, and some do mostly Y trainings. But it is a big organization and will vary association to association.
And, certainly if you want to teach someplace other than the Y you will want a national Pilates certification that will be accepted in the places you wish to teach.