I am just about to start taking it myself, however I couldn't find it anywhere in Hong Kong so I purchased it online from Australia at http://www.fertilitynaturopath.com/ They will ship it to Hong Kong.
Also, the correct name of the product is "Vitex Agnus Castus"
I was in a health food shop on Lyndhurst Terrace this afternoon (sorry, I don't know the name of it) and I happened to come across some Vitex. So, if you don't want to place an order online you might want to go there to get the supplement.