Sometimes I think you read my mind. I was looking at an affiliate link plugin in my files and thinking I should do a write up on it when I received a question on this topic from Cara Mirabella, of The Household Helper. Cara asked if I knew about WordPress plugins that would automatically add an affiliate links to specific key words in the article text.
My first experience was with WPAffiliatePro, which is not free or cheap either. However, it does have a lot of features. You can check it out here. Two others, which I’ve not tried since I have WPAffiliatePro, are WP-Affiliate and Amazon Auto Affiliate Linker. I’m not quite sure if WP-Affiliate does the same thing as WPAffiliatePro but I don’t think so. It seems to me like it is used more for cloaking links than creating auto links. The last one of course is for Amazon only.
Here are some more free plugins that you may want to check out. Before installing, make sure it’s compatible with your version of WordPress.