Must be step-down because ours is 240 volts.

Needless to say, it has to be a step down transformer because our country is using 240VAC standard (UK mains voltage) while that power adapter requires 100VAC (Japan mains voltage). I assume the power adapter comes/originates from Japan?

Get a transformer with a rating of at least 120VA or higher (preferable).

You can try online stores such as RS Components, Farnell/Newark/element14 and Mouser. Prepare to be shocked by their prices. You can get one way cheaper in places like Pasar Road (which means have to go out)...

You may try to find one in electrical hardware shop. they usually got sell one.

Mains voltage for Japan is very similar to USA, around 10V to 15V difference only (as USA uses 110VAC to 115VAC). Thus useable with USA mains power. Nowadays many electronic stuff are made in China.

You can see how the prices as like from online stores in the next paragraph. Then you can decide if its worth the trip or not. Anyway, this transformer is quite heavy (since I have one also, and quite big one), thus even postage or courier fees would be rather high. 

You have to check that out for yourself. More likely to find it at electrical/electronic hardware shops.

Actually they do sell, just you did not check carefully/properly. Examples here: element14 Malaysia: 172F - HAMMOND - ISOLATION STEP DOWN TRANSFORMER, RS Malaysia: Transformer,safety isolating,portable,650VA,110V,CMH1200/110 and Mouser: 172B Hammond Manufacturing...

Just make sure the wattage is correct (either equal to, or higher than 120W).

Mine not so small, and is rated for 500W (thus can imagine the weight also). If you want much bigger transformer then must manually search for it (in places like Pasar Road)...

Should follow the wattage on your power adapter (or higher wattage), otherwise may encounter low/insufficient power issues...

Looks like its rated for 300W, thus should be very sufficient for your device. You can also read the comment in the specifications concerning transformer longevity (which is why I've mentioned higher wattage as well). Also there's a caution mentioned there concerning devices with high current draw (ie. this part "Do not use it for hair dryer, rice cooker, electric iron, or any electrical devices that produces heat").

Why not go taobao, look for the kinect power adapter that is meant for china market, and get a china to uk pkug adapter. The adapter can get for rm2 or there abouts