A lot of people on RS like Kayak.com while personally I like using the ITA Software Matrix to do comparisons and ultimately booking with the airline directly. I never book using third-party websites. You can try a travel agent to see if you can save a few bucks but I would much rather have control over my own destiny.
Everybody's definition of "cheap" and "cheapest" is subjective so I would suggest you start with those two sites and go from there. You may also want to consider flying out of Bellingham via Allegient Airlines as some people have found really good deals through them however, I have never flown with them nor plan on to in the near future. Alaska Airlines also flies direct to LAX from YVR on a regular basis and their flights are pretty cheap however, I decided not to fly with Alaska Airlines for other reasons.
For myself, I prefer flying direct from Vancouver to Los Angeles on Air Canada where flights are typically $350-$400 taxes in. Continental Airlines is now a Star Alliance partner so you can also look on their website for prices and they co-share flights with Air Canada.
Just keep in mind the days you choose to fly can make all the difference to your end cost. I used to fly out of Seattle on Virgin America but after factoring my time cost, gas/mileage, parking expenses etc, it was cheaper to fly from Vancouver although I have to admit, some good deals can be had through them. YMMV. I'll be heading back down at the end of the month. Hopefully some deal/bargain hunters can chime in. Good luck!