Having a product link outside the content also works if it’s relevant. If you have a product page on your site instead of linking directly to the merchant, having the links will increase the search ranking of your product page.

Amazon is a popular and easy to use affiliate program. It is easy to link to specific products or they now have options that choose relevant products for the page and can also be based on the user’s Amazon history.

There are also affiliate networks such as Commission Junction, LinkShare and Performics. They have large numbers of advertisers and provide a common interface for getting banners and product links. Once you join an affiliate network, you have to apply to individual merchant programs. It is free to sign up for the networks and individual merchant programs.