The best way to find suitable brands to promote is by simply using a search engine using: '[Brand] + Affiliate Program'. Some companies run in-house affiliate programs however, this is a very specialist area. Therefore, most companies opt to employ an 'affiliate network' which has already built a large base of affiliates and gained years of experience in running successful programs for clients.
For example, if I talk about how cool a product is, and then you find out that I’m an affiliate for them, wouldn’t you as a conscientious observer become skeptical as to whether my information is biased, if perhaps I’m only saying how cool something is because I can get paid for it? Wouldn’t that make you question my integrity with other things I say as well?