I seem to recall that they're all clustered up north, so you're probably going to just have to make the trip. It's not that far - just walk a couple blocks west to the MTR station, and you can take the Tsuen Wan Line all the way there.
It's been awhile since I've been to Hong Kong, so I can't quite remember where the shops are. I wasn't really looking for keyboards when I went out shopping for electronics.