Very late response but a few things you need to know about that area.
San Bernardino and Angeles NF is in a very strict fire ban all year, this means no fires outside of fire rings and even no gas stoves in some places. All Yellow post sites have fire rings. The National Forest website seems to down right now but look up the latest fire danger release before you leave. Also in California to have a fire in a dispersed camp spot you need a cal fire/NF certificate, it's free and takes like 10min to get.
Permits
There is a few dispersed sites but not many because it is such a busy area the rangers try to limit you to the yellow posts to stop people making 1000's of campsites where ever they want. Most of the dispersed sites are to the north and north west Big Bear lake. On the south side along Skyline Dr is yellow site only. I don't know about the stuff even more south of that like Clarks Grade, Burns and Rattlesnake Canyons Trails.
Stop by the Big Bear Discovery Center on the north side of the lake, the rangers there are very helpful and have MVUM/OHV maps for the area that have all the trails and up to date camping info.
