for skateboards there are alot depending on where you staying..... i used one in queenstown and another at redhill .... just google it
for longboards i currently buy from Longboard Love ..... google it ..... they carry quite a few good lines includidng Rayne, Landyachtz etc etc ....... Spitfire Skateboard (google it) at Peninsula plaza also carry a good selection mostly Loaded and a couple of other brands
if u starting... i suggest you buy 2nd hand and try out first before committing to a new one
Spitfire skate at peninsula shopping centre. 2nd floor. Very close to sg camera store. Mostly for skateboards but i remember they do carry longboards too.
There are sizes available for the deck. U can check with the shopkeeper. Wheel size also depends on what type of skating he is doing. Protective gears are also available there.