You don’t necessarily have to have a huge site or lots of traffic. Consider emailing an affiliate program’s contact person (look for contact info on the site or in affiliate newsletters) if you send a lot of leads their way, rank well in the search engines for a related keyword or have a high conversion rate. Make your email compelling. Read my tips here. You just have to be a good fit and provide excellent value to the merchant. Another good resource for this is here.
When promoting affiliate offers, just make sure you are fully aware of all the terms and conditions attached to your affiliate program. Some programs can be strict about how they allow you to promote their products. For example, some may limit you to banner ads and links only, while others will allow you to use paid advertising, but won't allow email marketing.