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.
If you are out of stock and bouillon cubes, you can still proceed with the recipe. One option is to use a dry white wine. If a recipe calls for one bouillon cube dissolved into one cup of water, replace the cube and 1/4 of the water with 1/4 cup of white wine. Omit the other 3/4 cup of water, unless the recipe seems particularly dry.
Another option is to omit the bouillon cube, but still, add the water that is called for. If you have poultry seasoning, add one tablespoon of that to the water prior to adding to the food. It may not taste exactly the same, but the seasoning will help add some flavor.
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
Any homemade broth has loads more flavor and nutrients than anything you can buy at the store, and it's free of all of those fillers and preservatives, too. Start making your broth, and you'll never go back to store-bought. It's a lot easier to make than you might think.
A good way to encourage yourself to make homemade stock is to start a bag of vegetable scraps and store it in your freezer. Next time you peel a carrot, slice celery, or cut up an onion, instead of tossing the peels and skin, store them in a ziptop bag in the freezer. When you cook a whole chicken (or even eat a grocery store rotisserie chicken), save the carcass in the bag. The same goes for meat bones from leftover steaks, save them too, but in a separate bag from the chicken bones. If you prefer vegetable stock, toss any wilting veggies or herbs from your crisper drawer into the freezer bag instead of throwing them away.
Now you have a bag of stock ingredients. On a lazy weekend day, dump the contents of the bag in a stockpot, cover with water, and simmer. Your stock will be ready in no time.