There are two different kinds of transport cards: one that is valid for five years (and costs €7.50) and another that you throw away when you are finished (free). For most tourists, I assume, the throw away card will be more popular. These cards are sold at the GVB offices at the Centraal Station and in the metro station underneath the train station. However, they are also sold in automats at almost all metro stations in the city.

The offices at Centraal Station are mostly for buying the five-year tickets, getting information, etc. The automats are easy to find in stations (they are yellow and look like this: http://www.nieuwsbank.nl/pers/ccv/En...15-06-2009.jpg).

For more information, here is a page (in English) with the different kinds of cards, the costs and where to buy them: http://kwick.me/q5V3