You'll find a couple of stores in the airport (Schipol) that sell cards. The cost varies depending on how long you'll be there and how much you think you'll need to use the phone. I found the staff to be very helpful, very fluent in English, and you could choose to pay in US dollars if you preferred.

One note: I arrived early in the morning (as do many flights from the US), and the phone stores weren't open yet. Fortunately I was staying near the airport and could easily get back to get the phone card. I later found out that I could have gotten a phone card at the Amsterdam Central Station, or any number of in-city exchange offices that proliferate in the tourist areas (Dam Square, the Museum District, etc.)

Have fun--it's a great city!