So, here's the long story regarding specifically PROVINCIAL parks (as opposed to B.C. Rec Sites and Forest Service Sites):
There are TWO websites you need to deal with: One is the list of British Columbia Parks (http://www.env.gov.bc.ca/bcparks/); the other is the reservations website (https://discovercamping.ca)
One thing you need to know is that, while these are considered 'provincial parks', they sub-contract management duties to individual companies. Usually these companies collect payment directly, have specific contact numbers, etc.
The other thing to note is that many or most campgrounds offer both reservable AND FCFS (First Come First Serve) sites. The nice thing about the FCFS, is that there is a button on each campgrounds webpage on the DiscoverCamping.ca website that says "1st FCFS Expected Availability" in which they do their best to forecast the likelihood of availability at any given time.
So, what I would do is look at the BC Parks website to get detailed information on the parks, their amenities, etc. and use the reservation site for reservations, and to determine availability.
In my experience, the further you are from Vancouver, the more likely you're going to find some sort of availability.
Oh, one last thing, I am not positive, but I believe you cannot make reservations further than 90 days ahead. But you can confirm that yourself and report back to us.