Depends on what you want to draw. Mostly, when you think of a certain emotion, you already have something in mind. For example, a certain situation or color. From there on, it's easy to find the references you're looking for on google (or any other image site)
Start thinking of the ideas that you want to express and images that might express them. Don't go looking for reference at this point. Sketch out a plan for the picture you will make. Once you have a plan, then go looking for references that fit your plan. Don't change your plan to fit some photo you find. You may have to use yourself or some friends as the models so that you get the pose that you want. You may have to take your own reference photos or learn photomanipulation and compose different stock photos together to get a reference that you can make the final picture from.
The best advice I can give is to just let go, because expression in art is a lot of times done through suggestion. Find some colors and color combinations that in your mind represent the emotion you wish to show. Then, ask yourself how, for example, "Happiness" would look. Is it organic and full of energy all over the place, or a contained excitement? After that, let loose! Just throw color and shapes and form at the picture and don't stop until it's complete! Start small so you can get used to that kind of drawing, then work on getting larger. You can also always add actions into the picture that suggest the emotion you want to capture as well. Also, adding action will really bring out any emotions and expressions of ideas.
If that doesn't work, something I sometimes do is just wait until you feel the emotion you want to draw. It can be hard sometimes to draw emotions like anger or sadness when you don't feel them . . . so sometimes the best thing you can do is wait until you feel the same way the subject in your artwork does.