A bouillon cube is little more than dehydrated broth. When you drop a cube in a cup of water, it dissolves to create one cup of broth. So, if you don't have any bouillon cubes on hand, you can replace each cube called for with a cup of broth. Use chicken broth, if the recipe calls for chicken bouillon, beef broth, if it calls for beef bouillon, and so on.
If the recipe calls for you to drop a bouillon cube into the recipe without dissolving it in water first, you'll need to omit one cup of water from the recipe when you add your broth. This will keep the liquid volumes in the recipe the same.
Because bouillon tends to be rather salty, your recipe won't be as salty as was intended. That can be a good thing, if you're watching your salt intake. Just taste your recipe at the end, and add more salt to suit your tastes, if necessary.
Enjoy the Real Thing
Homemade broth has loads more flavor and nutrients than anything you can buy at the store, and it's free of all of those junky fillers and preservatives, too. Start making your own broth, and you'll never go back to store-bought. It's a lot easier to make than you might think. Here are some tutorials to take you through the process: