Sauternes is a commune of the Gironde département in France. It is also a wine region within the Graves portion of Bordeaux that produces sweet white dessert wines.
In France, there are several substitues for Sauternes, which is usually expensive. For instance, Monbazillac, Jurançon, Gaillac, and Sainte Croix du Mont (in front of the Sauternes region on the other bank of the Dordogne) are all excellent. Alternatively, you can also use Muscat, Moscatel (Spanish) or Moscato (Italian) sweet wines and red Banyuls, or even red port (the white port is too dry). All are excellent companions to foie-gras.
La vieille prune is an old prune distilled alcohol, most of the time proceeding from clandestine farm distilleries. Also an super alternative to Sauternes.