i use another red wine of a similar style if i can get it. if you have a library of already-completed kits, that serves as a good source, if not, a commercial bottle or even a good-quality bag-in-box wine is good for topping up. the 3L boxed wines are nice to have on hand for this because they will keep longer than an open 750ml bottle and you get often get several topping-ups out of a single box.