If you’re AFAA certified…you will find an “Agreement and Release of Liability” form in your AFAA book (first edition, 2006, revised 2007) in the Appendix C…page 429.
Also ACE has them at:
Hope this Helps!
There are many forms and examples out there to help you write one that suits your business. Try to Google “fitness waivers and liability forms” so you can get an idea of the information you will need to include in yours. I would also suggest you have an attorney take a look at it to give any advice that might be needed.