Hi Marvin, which area will you be staying? If you're Tsim Sha Tsui area, you can try at Harbour City Mall. Inside the mall at the Gateway section, there is a DVD/Blu-ray shop located in the 3F known as Hong Kong Records. The prices there are much cheaper than HMV video store. The address is:
Shop 3320, Gateway Arcade, Harbour City, Tsim Sha Tsui, Kowloon (Tel: 2175-5700) Other Hong Kong Records branches:
# Shop L2-01, Festival Walk, Kowloon Tong, Kowloon (Tel: 2265-8299)
# Shop 252, Pacific Place, 88 Queensway, Hong Kong (Tel: 2845-7088)
For HMV addresses:
# G - 1/F, HK Pacific Centre, 28 Hankow Road, Tsimshatsui, Kowloon (Tel: 2302-0122)
# Suite 101, 1/F Central Building, 1-3 Pedder Street, Central, Hong Kong (Tel: 2739-0268)
# Shop No. K3, 1/F Queensway Plaza, Admiralty, Hong Kong (Tel: 3692-4178)
# 1/F, Style House, The Park Lane, Causeway Bay, Hong Kong (Tel: 2504-3669)
# Shop F5-7, 1/F, Telford Plaza I, Kowloon Bay, Kowloon (Tel: 2756 3068)
Good hunting bro.