Sorry can't help with tickets.
But I doubt the ridebooker is the only option. Here are a couple:
www.pacificcoach.com
www.greyhound.ca
www.snowbus.ca
Pacific Coach is pretty much the same price as ridebooker (a couple dollars more actually). Greyhound is a bit cheaper, but perhaps not as convenient (doesn't leave from hotels, etc). Snowbus might be a good option to check out as well, and the rates look just a tad cheaper (not as late a return time though if you want to stick around Whistler after a day on the slopes). They do have some options for combo shuttle and lift prices though that might be worth a look (I have no idea if they save you anything).
I'm sure there must be other options as well, particularly if you can arrange to get to the airport and take some type of direct shuttle from there. But I imagine there are other options from downtown also.
I only suggested ridebooker because the price is pretty reasonable, and they seem to have a wider variety of convenient times - earlier departure and later return than Pacific Coach for example. They also have plenty of pick-up locations, at least around downtown. I doubt you'll find many lower price options for public transportation to Whistler from Vancouver during the Games.
I also expect prices for any options may increase at some point, possibly as soon as Dec 1. I think I read something about ridebooker possibly increasing fare rates in December (though I may be mistaken). It wouldn't surprise me to see the rates increase as we get closer to the event.