Garth,
Aren't single side swingarms, longer and heavier than the stock units?
Somchai,
Which country are you in? I found a shop has it, but I also don't know if they would ship to my country. I'd try one for $69.00 usd.
http://intijayamotor.com/?product=arm-kumis-ninja-250
The "Over Racing" swing arm is amazing, but has an equally stratospheric price of $720 usd.