I used to know someone that was a freelance trainer did not work for one specfic gym I think that would be your best bet for making some good coin. He once explained to me how it all worked but I forget the details was a few years ago, but I do remember him saying ISSA is one of the best certifications to get. I wish there were trainers out there that were not so full of shit. I hear and see some of them with there clients barely paying attention to there form etc. If I ever decided to get a trainer I would want them to be a form Nazi make sure I am doing every thing exactly right. It would also be great to have someone to spot me when I needed it sometimes I avoid lifting free weights to heavey cause I don't have a spot.
Good Luck