Karin’s points are great. Approach each gym separately and find out which ones are working with commission. Not all gyms allow outside trainers to come at their location, so do some research. Once you find the ones that you are interested at, check online to see what their policies are and proceed from there.
Also, working as an in-home trainer will give the opportunity to keep more money for yourself. You will have to be more creative, but at the end it will worth it.