If you'd like to use store bought stock for this recipe, I'd highly suggest getting the best quality that you can and definitely get low sodium or no salt added stock. This way, you can simmer it slowly to reduce the volume by about half and intensify the flavour before adding garlic, onions, nutmeg, splash of wine, herbs, peppercorns and salt to season. Simmer that covered for 15 minutes before straining through a fine sieve as serving as the au jus dip for the sandwiches.
Regular stock will likely be far too salty when reduced, so the low salt or no salt varieties will always work better.