For a person who has a Masters Degree in Piano Performance and also has a few years of teaching experience, what is a good place to get some sort of teaching certification that is accepted worldwide, preferably online. Now, I know this might not be the right place to ask this question... but maybe someone knows the answer. Thanks!
I don't know of any online certifications, but ABRSM (Associated Board of the Royal Schools of Music) do offer teaching diplomas.
This is the US site, but most of their exams are available in many countries and you will be able to find country-specific information on their website.
I highly doubt it's possible to get a teacher's degree online, cause you know, it's all about the practical stuff. As far as I know, you're only able to get a teacher's degree by doing it as a minor course during your bachelor/master, or do a post-degree teacher's course at a music college.
Note: this is how it works in the Netherlands, might be different around the world.