Back-roads Map-books, "Volume 4 The Cariboo" are about the most current on the market but even they are a few years behind.
The line between Zone B and Zone C follows the Spout Lake Road from Lac La Hache and winds up to the West of McIntosh Lake and eventually follows the Horsefly River to Quesnel Lake.
The line is not too clear in a few places and if you were planning on hunting close to the boundary I would contact one of the CO's in 100 Mile House - D'Arcy McPhee is the more helpful of the two - before your trip.