hey gucio,
you have a few opinions the cheapest one is
A) buy a stock rb25 turbo (from an r33 or r34) they are a direct bolt on if you have the later model waterlines, you need to an rb25 outlet elbo too as the 20 is too small
cost anywhere from $300 to $500 (depend model and km)
Then you have the aftermarket opinions;
Most common is the HKS GT seriers, being GT2530, Gt2535, Gt2540;
They are all bolt-on; cost anywhere from $1,000 to about $1,700.
Im not sure where u could findone in perth, i guess just check the forsale section, they are always for sale in japan (could get one for as cheap as $1200)
Hope this helps-
Michael