I don't know about Seoul, but in Busan you often get hospitals clustered fairly closely together and around them are tons of medical supply shops. That's where I would look first if you could find out where that area is in Seoul

There are plenty available online, seem expensive though, but I have no idea how much wheelchairs cost back home!

Link here:

http://shopping.naver.com/search/all_search.nhn?query=%ED%9C%A0%EC%B2%B4...

Both Gmarket and 11st have their websites in English if your Korean isn't up to scratch.