Haven't been back to HK since 2000, but I know there are a couple of game vendors in the Golden Computer Arcade (giant mall with different vendors). Shum Shi Po subway station, follow the subway signs/exits. I know I picked up an R4 variation there last time and they do older flashcarts as well. Check it out...I think different vendors sell different things. (I didn't ask for a flashcart, but picked up a copy of Monster Hunter 3G and the saleslady/owner offered me a R4i card for like $3 Canadian.)
There's a Wanchai computer market as well.