You could try making it yourself at home, it's so simple I couldn't believe I hadn't tried it before. I've never looked for skimmed milk in Korea, but would presume that it exists somewhere. 

All you need is one carton of yogurt with live culture. The Denmark yogurt available in most supermarkets is what I use. I won't go into all the methods to do it as a quick google search will tell you how. I use the low temperature setting on my oven, but a friend of mine uses a cooler imersed in a warm bath. A thermometer is also useful, but not essential. The temperature required is when the little bubbles form in the milk just before boiling.