Hia, this is a good one i think http://amsterdam.xpatrentals.com/
and for private landlords I have also used http://www.elynx.nl
I'd say those are the best two really, the other option is to ask a local to look in the classified of the newspapers or do it yourself if you can speak dutch. Its not easy finding the cheaper places in my experience as people tend to hold onto them for a long time.