There are several books by Kathy and Craig Maitland that we have found useful for finding BC forest service campsites:
Camp Free in BC 2005
Camp Free in BC, Vol. 1: southern BC 1999
Camp Free in BC, Vol 2: central BC 1999
Some of the information in the books is a bit dated and some of the sites seem like they where not visited first hand as occassionally the information is false.
In keeping with the theme of "camp free", I bought the books at Value Village for 99 cents each. Excellent value for the money