The only "certifications" I know of in any area of the performing arts are BA, BS, BFA, MA, MS, MFA and Ph.D. Theatre is not an industry like computer tech support with non-degree based certification programs like A+, Six Sigma and MCSE. Such programs require standards, and there are very few of those in theatre as I'm sure you know.
Actors have all kinds of classes that they can take but for the most part even they don't hand out "certifications." The closest thing we have to non-degree based training is shadowing, which has been covered elsewhere on this board, or an equity card, which shows that you're certified to work on union productions.
My degree never got me hired for a single job. Everything was networking.
If you're looking for some sort of third-party verified proof that you know what you're doing, make sure that your prior employers give you good references. Those will be used far more often than any objective proof that you took a class and passed a test. Make sure you keep in touch with those references year after year, be nice to them and ask them for referrals.
That being said, a few safety-based certs could be useful but won't make the difference between you and someone else in a competition for jobs: first aid, CPR, fire guard and the like.