Lasagnes I've had that I've enjoyed were both in Shinsegye. One was in the film themed restaurant on the 4th or 5th floor. The other was on the 9th I think.
I've heard Bella Citta on Gwangan beach is good for Italian food, but haven't been there so don't know if they have lasagne on the menu.
Most places sell the dried sheets. Why don't you make it at home. It's pretty damn easy.