i dont know where you can buy them new? but low mileage units pop up on ebay all the time, and occsasionally new holsets show up as well,...........
http://motors.search.ebay.com/holset_W0QQfromZR8QQsamcmZ6000QQsaspiZ2
I have seen some new ones that people use on l28ets, the 35 series, go for under $500