Hi Ken, you may want to check out a departmental store called Sogo in Causeway Bay. It is directly above the Causeway Bay MTR station. Head for the Toys section and they carry a decent range of Witty and HM models. Prices are pretty ok as well.
For smaller shops, head over to Mongkok district and just walk around Soy Street and Dundas Street etc. There is a Dragon DML megastore along Nathan Road (in Mongkok as well). 3 Stories of toys and models.
if you are looking for kits, then head to Waterloo Street (the stretch after the intersection of Argyle Street onwards)...plenty of small model shops for you to explore.
Enjoy ur trip
