I'd definitely leave from SEA -- perhaps you can get the bus from Downtown YVR?

Two other student places that I know of (besides travelcuts) are:
STA Travel and Student Universe (http://www.studentuniverse.com/). I don't know how much better these two places can do (doubt STA can do better than Travelcuts), but I would be tempted just to go for a published fare ex-SEA on the US majors.