You can get some good ones on ebay, but be carefull as there are some really shit ones around.
Do you want a really small one or a mid size one like mine?
MR R33 GTST
You can buy the supercrargers for the 4 strokes at a wrecker in dandenong.
You will need other mod's before the supercharger will work properly, and the stock carb, exhaust, intake etc and shitty