It is recommendable that you make hotel reservations well in advance. Because of other summer events in Amsterdam, a shortage of accommodation can be expected.
Some hotel booking sites:
http://www.booking.com/city/nl/amsterdam.html
http://www.hotels.nl

Amsterdam

http://wikitravel.org/en/Amsterdam: This is a general travel site about Amsterdam. It contains up-to-date information on travelling to and within Amsterdam, trips in and around the city, places of interest (museums, parks, etc.), restaurants, cafes, clubs, shopping, as well as accommodation of all ranges. Transportation
Information about travelling to or within Amsterdam can be found here.