For 3rd party lenses, you can try yu's camera. Their address is:
They have really good price especially for grey stocks.
other shops I recommend are wing shing and man shing. Their prices are very competitive as well. Try to avoid tin cheung, been there recently and their prices have went up by a lot since the past few years. You can also try this mall call 'sim city' there's quite a few camera shops there.
one952: haha yeah, eat and shop there mostly. hope can pop into some shop selling cheap pentax gears
pentriot: thanks for that link!
souloner: thanks your for recommendations. will check those shops out when i'm there!