There are lots of Amazon affiliates in this space, mostly because it’s a strong hobby (i.e. passion) niche with tons of products. Which is good, since tons of these big stores are very general in nature, making it possible for big outdoors sites to possibly even pick one or two stores to recommend (this, in turn, might have other advantages, too, like negotiating better rates with those brands).
When do you want to get started? If you are just starting and have no audience then some of the programs may not accept your application to become an affiliate, while others will let you get set up in minutes. If you want to get started straight away then make sure you’re applying for programs that are less restrictive. Generally the larger, network style places will enable you to get going quickly while specific niche programs may require you to have a certain audience size or characteristics.