To my knowledge, these are rarely, if ever, sold as spares. Depending on the headset, you might get one by emailing the maker, but most likely you'll need to do without. The lower seal is more important than the upper, so if possible switch the other one down. Then improvise a seal with an O-ring, if you can. Or you can fit an exterior seal with a wide rubber band cut from an inner tube or using the one they use to hold broccoli together.