Hi Che, as the D200 is newly launched and a model that many were anxiously waiting for, the chances that it is out of stock is high. This being the case, I would suggest you call the shops and reserve a unit for when you arrive.
John 3;16, as suggested by Ahv is in Funan, and near other malls which have dedicated photography equipment shops. Just a walk across the road is Peninsula Plaza. In it you can find a number of these shops, Cathay Photo being the biggest, and offers the best deals (within Peninsula that is). As these malls are around the City Hall area, establishments nearby include Chijmes (restuarants, pubs, cafes some shopping), City Hall shopping complex (within the Swissotel hotels, formerly known as The Westin Stamford and Plaza, S'pore's tallest hotel), The Esplanade, The Fullerton Hotel, The Supreme Courts, etc. As a visitor to Singapore, if you've never been , this may be a good time to just walk across and check them out. All are within a max walking distance of 30mins.
Alternatively, if you're here to just grab the cheapest D200 and go, as far as we know, the cheapest place is MS Colour located at Block 711 Ang Mo Kio Avenue 8. You may grab a train (MRT) from the airport and alight at the Ang Mo Kio station, and it's just minutes' walk away. This is at a small town centre in the heartlands so don't expect posh layouts and people. But I must add that the service staff at the shop are friendly and very helpful.
Call them now to book your D200 at +65 6457 6380.