The difference between stock and broth is very simple. Stock is made using the bones, while broth is made using the meat. In the stores you are going to find mostly broth. They are just starting to make stock, but I see a lot more broth. I know swansons sells a stock product. If you want to make your own homemade stock it is not that hard, and homemade stock is so much better than store bought. I do not make much homemade stock, but that is only because I do not eat or cook much chicken so I do not have enough bones to make stock. They do sell vegetable stock, so if a vegiterian is coming over you can still add lots of flavor to the couscous.
If you want to know a basic stock recipe I can give you one.