VNT's are not widely available commercially, mainly due to them not waning to take the risk of tons of warrenty cases caused by mis-use.
Ebay is the place to be, esp Ebay america, been a fair few VNT GT40s on there recently, never mind smaller ones.

IMO unless your an expert (dont seem like you are) and got money to burn experermenting (dont seem like you have) id stick with conventional turbos.

What spec is your current engine? Built for 400bhp dont mean much.