You can find plum extract in most major supermarkets across Korea and other Asian stores. Some supermarkets in the USA sell this in the international section, or you can visit Asian specialty stores for this product. If you can't find any plum extract in stores near you, you can always try online stores like Amazon US. Alternatively, you can make a homemade plum extract with Korean green plums, though these fruits may not be readily available in your area!