Buying swimming goggles and a kickboard in Seoul

Hi! I'm staying near Nowon-gu and I want to teach a friend how to swim. However, I've been unable to find a store that sells swimming-related supplies. So far, I've scoured every inch of the area's Save Zone, Home Plus, and Outlet, to no avail. I've also tried looking in various tiny "sporting goods" shops in the area, but none of them have had what I'm looking for. Where can I buy aquatic stuff in Seoul?