There is an excellent book that has just been published called "Camp Free in BC" by Kathy and Craig Copeland, aailable at most bookstores and at Amazon. It's a great resource for Forest Service sites and others that may charge a fee in peak season but are free otherwise. Very detailed instructions on how to get there, what is available, whether suitable for trailering, whether you need a high clearance vehicle and so on. We have found some fantastic free spots in this book. The same authors have other good guides including hiking in BC and the Moab area. Here is the authors' website:
Craig & Kathy have hiked many of the world's mountains and canyons