I know of the postcards you are speaking of. They are sold in a box of 40 and were put out by Bryan Burkhart about the same time as his "Land Yacht" book in 2000. Airstream used to have them in their online store, but I don't see them there now, I didn't spend much time looking, but they didn't jump out at me.
I think what people are doing is buying the set and breaking them up into individual cards or small lots to sell on ebay. You can purchase the whole 40 postcard set
from AirstreamDreams for $12.95....individual cards can sell for a buck or two....so do the math. What I don't like is the ebay sellers that pass them off as "collectable"...that is deceptive, but then again it is ebay, "Buyer Beware!" Besides, how "collectable" can they be if there are so many on ebay?
I have a set I got a couple of years ago as a gift...they are nice and we have sent a couple to family while out & about, but most of them just sit there in the box. I won't buy more if I ever get through these because it is a "been there, done that" sort of thing now...over saturated, everybody has seen them by now.
There is also a new set of 20 notecards
offered for sale at Airstream Dreams based on the newer red Travel Trailer book, also $12.95...not Airstream specific but look nice, although I haven't seen them in actuality.
P.S. no, I'm not one of the postcard sellers on ebay...