If you’re ACE certified, check out the Personal Training Manual. There are plenty of examples of liability waivers, PAR-Qs, health history, and assessment forms in there. If you wanted to write your own, I would suggest googling “liability waiver”, and see what you find. Just make sure you read it completely through, tailor it to your business, and maybe even get a lawyer to take a look, just to make sure you’re completely covered!
Hope this helps, good luck!