From personal experience, the best way to find a room (in Amsterdam) is to just ask as many people as you know. Because many of the best-priced rooms – especially for the good locations in Amsterdam – never go on the market. That is why I recommend you to even approach friends of friends as a housing source!
So get in touch with all your friends and connections in Amsterdam and ask them if they have friends or know someone who rents out a place. Since I’m not sure if this is the answer you’re looking for, as this isn’t a concrete offer, I hope my post has an optimistic tone, because if you’re looking for it and using all your tools to the fullest, you will find it eventually. But yes, finding a place in Amsterdam is definitely a full-time job :))