You need - at minimum - three different merchants to work with. Never put all of your eggs in one basket, meaning don't rely on a single merchant or a single affiliate network. "Stuff happens" (said from personal experience, lol) so you want to diversify and have backup merchants & networks to work with.
The days of building "Adsense sites" or "Amazon sites" are long gone. You need a variety of merchants and networks, and even product types, to secure your business and truly serve your market. The key is in finding the right products and the right affiliate programs. There are several things you need to consider...
How to Find Affiliate Programs
Finding affiliate programs is actually easy. You can go to Google and search "your keyword + affiliate" to start. I have a low carb blog, so I would search that:
You can also search your main keyword in affiliate networks such as Commission Junction, Shareasale, LinkShare, ClickBank, etc.
Another option is to go to Amazon.com and search your keyword, which will usually give you a variety of products to consider. Obviously Amazon has their own affiliate program, but I often do this just as an idea generator. You'll see a variety of both physical & informational products (books), which should spark some ideas.
In that Amazon list I see Quest Bars, so I can then go to Google and type "quest bars affiliate" and find their affiliate program directly.
I tend to promote products I use, so I always look for an affiliate program for those products specifically. It really helps if you ARE your target market, because you can simply look at the products and services you purchase yourself as a starting point for what you'll recommend to your audience.
TIP: create a Swipe File of resources and products/services that interest YOU. The blogs you read, videos you watch, forums or groups you join, products you buy, services you subscribe to, etc. Your swipe file will be a goldmine for both content ideas and for products to promote.
Recommending Products You Haven't Tried Yourself
For the most part I at least test products I'm going to promote, though most things I recommend are something I am personally familiar with.
That said, I'm not against promoting proven brands even if I am not using them myself for some reason. In some cases I go with big name brands that are trusted anyway. Amazon aside examples include Ticket Masters, Wal-Mart, 1800Flowers, etc. It depends on your niche obviously.
Given I *am* my target market in the niches I work in, that makes it easier. As an example I promoted Curves (a gym for women) on my low carb blog last year. I'd never been before. I actually used the coupon I was promoting, and signed up for the free week. I blogged about it, and even did a video about it, which really helped conversions.
Your market wants to hear about personal experiences, and your enthusiasm and/or results will encourage them to try it themselves!
Physical Products vs Informational Products
Don't get hung up on selling information products just because the commission is higher. In most cases, physical products actually convert better. It also depends on your market, and of course the keyword phrases you are targeting. Don't try to sell information to someone that wants to purchase a physical product, and vice versa.
Keyword Research will tell you a lot about what your market actually wants to buy. But if you want to see if they purchase "information" you can always test that by promoting a book from Amazon, or a low cost informational report, prior to creating your own information products to sell.
Proven Products vs New Products
Many people look at various metrics to find affiliate programs that already have proven conversion rates. There ARE benefits though, to working with new merchants or products that don't yet have stats.
For starters, newer merchants may be more willing to work with you on unique angles or exclusive discount codes. You'll also be informing your audience about new products, which is a great service to your market. There is also less competition with newer products, especially before they accumulate any real stats.
What If They Don't Have an Affiliate Program?
Sometimes there are products you want to recommend, but there is no affiliate program for it. Contact the merchant directly to inquire if they have a private affiliate or referral program, or if they are interested in working with you directly.
You may also find other merchants that carry that brand, or that particular product, such as Wal-Mart or Amazon or other online stores.
Anytime you can work directly with the merchant it's best, but sometimes they aren't open to it. In that case, simply search for the product on Google and find out where else it is sold.
How Do YOU Find Affiliate Programs To Work With?
The tips above are from MY experience as a Super Affiliate. I would love to hear some of the ways you've found and/or selected products to recommend online.
p.s. Recommended: The Super Affiliate Handbook by Rosalind Gardner. It is currently on sale for only $27, which is a steal. This was the guide that turned ME into a Super Affiliate.