Pioneer Center in Prince Edward has two big diecast airplane retailers. One of them even have a sub-branch in Hung Hom as well. Here is their website
http://www.westmaintoys.com/
There is another one called Toymax in Jordan, easily accessible through Jordan MTR station as it is connected to the shopping mall Toymax is in. The whole path is enclosed.
http://toymaxco.cc/
But there is a 99.9% chance that Toymax won't carry CX models, and as for the other two at Prince Edward, they probably won't be selling you CX models either unless you are a regular customer.
Go onto the internet and look up 80M Bus Models. They have a chain of stores around HK and specializes in buses, but they do carry CXcitement (name for CX duty free items) planes. It's just that the quality of CXcitement's models varies between aircraft type. Some are decent, others are a bit mediocre. CXcitement models (manufactured by Herpa or Hogan) has pretty good 777s and 747s (apart from the odd looking nose shape), but of course there are better moulds out there if you want to look beyond the officially licensed models... It's the primary reason why most diecast plane retailers around HK will be reluctant to sell you a CX model if you are a new customer.