You can buy canned or boxed chicken stock at the store, for one. This isn't the best option, as the taste isn't great. What I do is make my own broth from leftover chicken bones, onion, and celery. This is the best-tasting and often cheapest option. Here is an article with an extensive, semi-scientific explanation, but the process is really quite simple. It ends with this recipe. Somewhere in between those options is Better than Bouillon soup base, which you can mix with water to make good broth quickly and fairly cheaply. I personally use this when I don't have the really good homemade stuff on hand.