There are a lot of ad networks that run fitness offers, letting you promote supplements, apparel, sports gear and even entire workout programs or weight loss systems.
With fitness CPA offers it is important that you have a clear target audience in mind. Note that what will work for one niche won't always work for another.
If you are trying to reach athletes, you need to market something like the P90X program, which is challenging and will scale to different levels. For those slightly less athletic, but looking for something really fun and that will give them a nice buzz, the Beach Body or Insanity workouts are good. For those who don't want to work out at all, fat burners could be the answer. Then, there are people for whom weight and fat is not the issue. Climbers and martial artists might want grip trainers. Bodybuilders will want preworkouts and protein shakes. Crossfit enthusiasts will want shoes, resistance bands, t-shirts and other accessories. If you are looking for a steady, sustainable income, then fitness CPA offers are a great option.
Every niche has its own interests, and its own markets to be discovered. The challenge, for you, is to figure out what people are interested in and to find an audience that will buy.
It helps if you can talk genuinely and passionately about what you are selling. Talking about your own experiences is easier than regurgitating studies. It will also give you the chance to post some cool before and after photos. Plus, you will know what you are selling is something that you believe in. People who are serious about fitness can spot a faker, so it really is important to be honest and to know what you are talking about when you sell something.