I *think* you can only get them at Central Station, but you can get a train from the airport straight into Central Station and then you're right in the city centre as well. They're cheap too, I think we only paid about 14 euro for the 3 days unlimited travel. Get to know the trams because Amsterdam is the most confusing place to walk around. Nothing is in straight lines and all the streets look the same.
Seriously BTW I can't stress how much you need to prebook. I went to the Van Gogh museum at 10am on a Sunday (with a roaster of a hangover) and there was about 200 people queuing outside. Everything is so so busy.
Check Paradiso for gig listings too. They always seem to have something cool on. When I was there they had the Julie Ruin in one room and Frightened Rabbit in the other.