Are you either a certified personal trainer or group fitness instructor? If you are, you can incorporate tabatas into your classes or with clients without having a separate certification.
Tabata is an intense interval format, and as long as you understand the format and your client or group and their fitness levels, you should be able to use this format safely. I teach tabatas frequently as part of my bootcamps and interval classes.
That being said, if you are looking for a certification, I’ve heard that Mindy Mylrea has a good one–see www.mindymylrea.com for more info. I’m sure there are other programs out there as well, but I don’t know the quality of them.