I've found the most success with affiliate programs that are in alignment with things I'd be talking about organically. I'm typically writing/publishing about various topics at the intersection of photography and technology so my affiliate partners include a range from photography-specific gear companies to online services that would be of use to photographers. Look at your most popular articles and topics, and consider what companies are folks that you're already mentioning or that would complement what you're talking about.
When in doubt, it can't hurt to reach out directly to a company and ask if they have affiliate or partner programs. In general, the bigger your reach, the more likely they'll be to want to work with you.
As to sites where you can browse and find affiliate programs, check out ShareASale7 and Commission Junction.4 For general products/merchandise, everyone seems to love Amazon3. And in the interest of disclosure, those are affiliate links. 🤪