There are a few places you can find roommates in Amsterdam.
Though I must warn you that it will take some work (replying to ads, posting ads yourself) to pull off, unless you are very lucky that is. Also, please be very wary of scams, many people prey on the desperate and are very cunning. So never send money before you have seen the place and met your room mates.
Kamernet.nl has lot's of rooms going by daily, you have to pay (±€20 minimum 1 month) to be able to answer advertisements, but it can be worth it because you get directly into the room share market.
Facebook has many groups, it goes fast there, but I have found my current place by reaching out to someone posting it there.
Here are a few groups to join, there are many more:
* Woonruimte Aangeboden / Gezocht (Amsterdam)
* I Need a Room! Rooms/Roommates in Amsterdam
* ROOM AVAILABLE IN AMSTERDAM
* Housing in Amsterdam
* Amsterdam Flat-hunting - find a team instead of fighting for rooms!
One last thing: Many people are sub-renting from sub-renters, it is common that they will not let you register there, but this is required if you plan to stay for more than 6 months, or need to open a bank account, I would make sure you find somewhere that lets you do this.
Best of luck!