I sent a lot of postcards when I was in Amsterdam. Including
my side-trips to other places in Europe, I sent 118 postcards.
Basically,
one a week to three places: to my wife in Waterloo, to mother and
sister in Pasadena, and to my sister and her husband in San Diego.
I also sent a lot to a friend in Boston, and random ones to other
people.
You can get postcards through-out Amsterdam. The prices are all over the place. The cheapest place I found was at J.v.Beek B.V., a smoke shop on Damrak that sold some for f0.50 for their cheap cards. The nicer cards will cost more, but at some places the cheap cards are expensive, too. The shops outdoors in Museumplein have similar prices. On the other hand, if you get them at the shop in back of Westerkerk, expect to play f2.50 per card!
Postage for internationl postcards is f1.00. You can get stamps at most places that sell postcards, but they'll sell you the boring, standard stamps. You're much better off going to a postoffice and buying stamps there. Alas, the city stamps (f0.80) were the nicest (it's f1.60 to send a letter outside of Europe, so I could use 2 f0.80 stamps) and I didn't see if they had f0.20 stamps. However, the f1.00 stamps at the postoffice were nicer than the ones at the postcard shops.