I've seen some Ferrari stuff at Sogo (Causeway Bay) before. They have a small consignment shop in the toy section (6/F). Not sure if they are still there or not.
Toys R Us has a really fast Ferrari remote control card with lights and rechargeable battery pack. Costs around $350HK. Ladies market in Mongkok sells ferrari shirts and jackets in some stalls. (not the 'real' deal but they're cute)