I make hard cider from time to time. Like Joe said, you can buy yeast nutrient at a home brewing supply store, particularly one that caters to wine makers. If you don't want to do that, a cider maker's trick is to add a handful of raisins instead. They'll release nitrogen into the wort as they break down.
Whatever you do, don't just skip it. During fermentation, the alcohol produced by the yeast helps prevent other nasties from growing in the wort. If you leave out the yeast nutrient, then your wort will be sitting around at room temperature for a long time without a lot of protection. You can end up with ginger beer vinegar due to bacterial action.