Vishnu
http://www.vishnuperformance.com/
Ludespeed
ASR
Edit: Rallispec
http://www.rallispec.com
JC Sports (but just on word of mouth alone I do not recommend dealing with them.)
There are a few others as well that I do not recall off the top of my head. I'm sure others will fill in the blanks with URLs and company names. You might also want to search around the turbo forum. Hope this helps.
[This message has been edited by closure (edited April 12, 2001).]