Scott...I saw a vaccum pump going for 325.00 on eBay for a w140. Thats not exactly cheap, but its a heck of alot cheaper than getting it from the dealer.
I go into eBay on a daily basis and do a search using "w140" in the search field. Brings up most of the items that will fit my S320. You can also try "mercedes pump" or something similar.
1998 ML 320
1994 Blue C220
Im going to Grad School @ the University of Iowa