I think you are talking about this kind of set:
http://item2.gmarket.co.kr/English/detailview/item...If so, those are Northern European mattress pads, not to be confused with mattress toppers. Mattress pads are thinner and attach with elastic bands diagonally at the corners of the mattress. When you see them in cute sets they are part of a "collection" unlike a bedding or comforter set in the U.S.
You can buy them quilted, heated, waterproof, stain resistant...etc. In Europe, beds are made with a fitted sheet and comforter inside a duvet. By adding a mattress pad above the fitted sheet, they can accomplish what a mattress protector does in the U.S. that is put underneath the fitted sheet. Flat sheets are rarely used in Europe.
I noticed in Jealousy Incarnate that the t.v. station dorm rooms had beds fitted with the zippered mattress pads that cover the whole mattress and act as the fitted sheet and mattress protector in one.
If you search for quilted pads and quilted pad collections, you might find exactly what you are looking for online. You'll also find a lot of the standard white toppers and protectors that are popular in the U.S. Mattress pads in the U.S. tend to be used for crib bedding.