I have the ATE T51 ("tii") brake boosters for sale now that have been professionally rebuilt and already adapted for the e9s. They are $650 plus shipping. And, yes,
@aguirola is correct, the days of less than $500 ATE T51 boosters are long gone even on 02FAQ. Also, there are many sellers claiming that they have used boosters in "good" condition that haven't even been vacuum/poppet valve tested. Buying a used booster is always a crapshoot, so caveat emptor.
An historical aside: the "tii" brake booster was first used on the 2000CS, the 2002ti and other BMWs before being installed on the 2002tii and 2002 Turbo.