If you can't find the larger packet of brown gravy mix (one that makes 2 cups of gravy) then you can use two smaller packets.
Find the gravy too thin? Simply mix 1 tablespoon of cornstarch with 2 tablespoons of cold broth or water in a small bowl. Stir the mixture it into the slow cooker. Cover and increase the heat to high for about 20 minutes or until the sauce has thickened.