there are some certifications on the internet where you only need to answer a series of questions (very easy from my point of view), then submit them, and if you are willing to pay their fee immediately, you are a certified personal trainer right there and then. In the same spirit, you can become their master trainer and what not. – You probably gathered from my tone of writing that I am not overly impressed with that process but it is available and out there.
Most certification organizations such as ACSM, NASM or ACE to name just a few will not make it that easy for you. I would hazard a guess that – no matter how experienced you are – you will not be able to pass their exams without studying their material (but I may not do you justice here).
The decision will ultimately be up to you. I would choose a nationally recognized organization of high standing in the industry for myself but your goals and intentions may be different.