Niches for Affiliates are not longer available on the internet if you think in terms of Search Engines.
If you check CJ or Linkshare all merchants are involved in high audience products.
Maybe for a product owner, if he has a very special stuff, there are still niches availables.
If you want money from Aff. Programs you have to face a fierce competence.
It is the same for casinos, books, marketing, hosting, travel, sports etc.
To make money in this industry you have to sell products in high demand.
Building a website around a hobby could be great.
Mainly they profit from contextual advertising (Adsense ans others).
But anyway to make some commission profit you need a huge audience.
Not a niche audience.