i amsterdam card

It can be purchased in Amsterdam. The best place to purchase it is at the tourist office at the airport. The TI at Centraal Station suffers from very long lines. In addition you might spot folks walking around the airport, train station, and city center wearing a very noticeable "i amsterdam" uniform...it can be purchased from these folks as well.

http://www.iamsterdam.com/en/visiting/iamsterdamcard/purchaseyourcard

Just make sure you're going to get your money's worth out of it. In 2008 I purchased a 3-day card (after waiting in a long line at the VVV in Centraal Station) but I should have saved my money. I was in Amsterdam this past July and didn't buy a card. With the card, the free public transportation was nice and I took a free canal boat cruise, but the card could not be used for the Anne Frankhuis, one of 2 museums I went to while I was there. (Maybe that restriction has changed now.)

(FYI - As far as public transportation goes, you can now catch these little blue buses (1 euro for a 2-hour window of time) that follow a route around the city. The route isn't as convenient as most tram routes, but the buses will get you within a 10-minute walk of Museumplein and other tourist destinations. A lot of locals take them also and you can catch them at Centraal Station.)

we bought one at the TI at centraal station and loved it, though no we did not get our 'money's worth', but then again we were not planning on it. we mainly bought it for ease of transportation, which it definitely provided. we bypassed a line at the van gogh museum (which, given the line, was entirely worth the euros we spent on the pass), and used public transit quite a bit. additionally, we used the coupon for a free canal boat cruise, though we wished we would have paid for one that was open-top (the two covered by the pass are glass topped, and sitting 'inside' is not the best view).