there are quite a few sites that you can search that will likely give you the best prices available. check out the website BookMe and you will be able to compare prices on all the reputable travel sites in just a couple minutes. it s very convenient and will save you a bunch of time and money. Also, make sure you buy them online... otherwise they charge much higher fees for phone orders. good luck.
if you want to enjoy the beautiful view on the way, then by car. I travel a lot and i actually use laxcarrentallosangeles.com their prices are amazing and they actually have holiday deals for even cheaper prices. you should definitely check them out
Amtrak might be pretty pricey, what you can do.. Fly from LA to Seattle via Virgin America.com -- can be as low as $ 69 (before tax) each way-depending when you go? Once you get to Seattle take bus or the ferry to Vancouver.