and here is one in MongKok :
www.modelpower.net
You can see the address at the bottom...actually it's near WAT Models.
And there is another one in Sham Shui Po area (next to Mong Kok Area), name and address is
Fok Lee Model Shop
Address: Ground floor, 66A, Un Chau Street, Sham Shui Po, Kowloon
Indeed this shop is also near the famous Golden Computer Shopping centre and you have to go early as this shop closes at 7:00pm everyday.