Go through the math textbook and look for appropriate story problems. While each textbook is different, every chapter contains at least a few story problems that you can give to students. You can also go back through old tests and quizzes, pulling story problems out of those past endeavors and bringing them back to make sure that students can do them more than once.
Go online and print out sample quizzes and test questions. Websites like IXL, Tulyn and Soft Schools all have story problems appropriate for a third-grade class, with the questions split up by math subject so that you can pick and choose which ones you want to give your class.
Create your own math word problems, and assign the creation of new problems to the class as homework. Create situations in which students need to total up the number of students based on gender in a class or make change for money, or even allow students to find the area of the gym floor. Anything like this can become a story problem that will get 3rd grade math students involved.