Citric acid is a common ingredient in cooking, and is used in making preserves and jams. You may be able to find in at your local grocery store. I know that I substitute it for lemon juice when making jam, but as I have not make the mozz recipe that you are looking at using, I am not sure whether it can be used for that. Maybe someone else can advise you better than me.
Best of luck and let us know how it turns out.