everyone said was terribly expensive due to it being a
wierd connector.
 I ordered it from 1-800-digital and they want $58(us) + tax.  For those
 with 3100 model 40s (it changes from model to model) the part number is
 17-02-008-02.
 
 I'd also like to point DECheads looking for cables to a place called
 SeaCoast Digital, at 
http://www.seadec.com/.  They have some dirt-cheap
 prices on DEC cables.  I'm sure you -don't- want to hear this now, Jim,
 but I've ordered the exact same cable through seadec myself, and it was
 $28.00  (sorry!).  They're a very good source for that kind of thing.
 I can't vouch for their other parts, as I've not compared prices for
 memory or option boards etc.  But definitely check them out if you need
 cables.
 -Seth