this is an interesting idea. You launched this business only recently . It would seem to me that you are confined to a rather small geographic area; I saw that you want to audition the instructors personally.
Since you promise that you will provide an instructor “last minute”, how confident are you that you can so that on the assumption that studios and clubs will usually try to find a sub within their own ranks. You would need a substantial number of instructors who sign up to be a on your sub list to have a chance. You (or somebody) would also need to be available pretty much 24/7 to field requests and disseminate them.
It looks to me like an awful lot of work for not much ROI. You can only expect a fee lower than payment for a class (which is often not much to begin with). YOu may end up being better of driving to the club and subbing there yourself.