BoardGameGURUPrince is available indefinitely at the BoardGameGeek Store for $5 as well as the other 5 promo cards: http://boardgamegeekstore.com/search?q=Dominion
Though you don't have to order from 2 places ... unless you already have Prince ... or don't have the rest of the promos.
Sooooo ... meh. Depends on what you need.