Watch Ebay: (search) Airstream: Parts & Accessories. There was a fellow who was parting out an older rig a few days ago, and he had a sink. Here's one:
eBay Motors: AIRSTREAM STAINLESS STEEL DOUBLE BASIN SINK OEM (item 320231086212 end time Mar-29-08 12:47:13 PDT)
Others come up from time to time.
Lynn