Try www.sidestep.com, www.kayak.com, www.statravel.com. I’ve used itasoftware.com but don’t trust them. In my experience, the fares listed are never the current so when you call them it’s always hundreds of dollars more than what you saw online.
 
I don’t know your dates but from 15 august to 15 september, playing with www.flyglobespan.com you can find fares into london for as low as 600US$ then take easyjet for about 100US$ roundtrip. Flying into Ireland or England first is usually a good option for US people so you may want to try it.
 
Good luck!

I tried Air Transat ($790 CDN cheapest), Orbitz (Martinair $840 USD), Martinair.ca, Herman.ca, Flights.com, ITA, and Travelocity.ca. Farecompare found Martinair at $830 (USD).
www.airtransat.ca — looks like the winner this time

Of course the usual applies – there are numerous cheap flights from Toronto (discussed on other threads) to various destinations in Europe and the UK, and from many of these you can get to Amsterdam pretty easily and cheaply.

thank you to all of your for your suggestions.  I think Air Transat does win.