If you go to Fastlane (top of this page) you can find a new pump cheaper:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41SR1A6G3G&year=1992&make=MB&model=300-DT-003&category=All&part=Diesel+Vacuum+Pump&appEngines=_any

Give them a call to see about overnight delivery.

Other than that, try the local salvage yards where you are. Hopefully someone will have a used one available.