Last week we bought a pretty good Korean mat (roll-up, one piece, thick, with a non-slip backing) from a toy store on the 5th floor of Great Century Place (mall connected to KCR Mongkok East station). Pretty decent quality, 1.8m x 1.4m, HKD500. Sorry, I don't remember the name of the store, but the mall isn't that big so it shouldn't be hard to find, the mats were on display standing at the entry to the store. They also sold 2m x 1m Thomas roll-up foam mats for under HKD200. The 5th floor of Great Century Place is full of stores for babies/kids (clothes, toys, books, games, strollers, maternity goods etc.)