try "gateway" in central. they normally have stock on diapers etc. make sure you know your prices before you go down there to buy. you can try pampers direct. try to get the contact by joining the pamper's baby club which is offered through watson's. they will let you buy in bulk but not at any substantial discount or discount at all. you may be able to get them to deliver though.
4T-5T easy ups are not exactly what i have seen here in hk, but you can find the pampers local equivalent then get yourself to a watson's/mannings. i have found that going to a local dispensary (drugstore), they have the supersize diaper packs and are much less expensive than the watson's/mannings shops.
good luck.