The gyms/clubs want Certificates for insurance purposes, but, the clients generally don't care. So, if you want to get certified, I would contact gyms in the area and see what they recognize.
Well, yeah, most clients don't know anything about certificates, etc. Its all for liability, in case the gym gets sued. But, if you can help people reach their fitness goals, that is all they care about.
I think the issue becomes when you train someone long enough, they reach their fitness goals, and learn enough to train on their own. They then no longer need a personal trainer. Thus, you're assuming that there will be a revolving door of people.
I think its also better for a gym to have sales people that recruit potential clients for the personal trainers, instead of having the PT's do it themselves.