With a small site that you're likely going to run yourself in your spare time, you won't be able to keep the site updated with fresh content and be effective at handling sales—especially not for a site that you're just looking to make some extra spending money with. Trying to get your site in front of potential brands that might be interested in advertising on your site is difficult and time consuming. So focus on the content and skip the sales by putting ads in from Google AdWords.
Google is where a large percentage of sites get their ads. Simply sign up at Google.com/AdSense , and a few short forms later you'll be good to go. Google will be able to give you pretty much any size display ad that you want, and will run ads on your site without requiring a minimum number of visitors—which is one of the reasons so many sites use them (although if you want to use Google for video ads as well, you will need to prove a certain level of monthly traffic). With Google, you probably won't be able to expect the highest ad rates, but you can be confident that there will almost always be an ad available when your visitors come. And overall, they're pretty easy to use.