At this time of year you could probably find somewhere when you arrive but that still means dragging your bag around the streets, looking for somewhere, when you could be spending your time more profitably. I'd suggest booking in advance.
For the full accommodation list on Amsterdam's official website go here:
http://www.amsterdamtourist.nl/en/home/to+stay .aspx
For cheap accommodation in Amsterdam, go here:
http://www.hostelworld.com/
Enter the relevant information and click 'Additional search options'. Put check marks alongside 'Budget Hotels' and 'Guesthouses'. The resultant list will show an average customer satisfaction score for any hotel which has received at least 6 or more reports in the last six months. Even when fewer people have subitted reports you can still access any customer feedback by clicking on the hotel details and then on the 'Ratings' tab. (Note that the prices shown on the main list can sometimes be rather misleading. They often refer to accommodation in multi-bedded rooms. You might have to hunt around for a while but you should find something which meets your needs).
Chris