You should be able to find it any of the forests in the area, It's not protected so it should be fine to pick but call your local forest service station and check just to be sure. Look for stream and river beds as these have more trees, it generally grows high in oak trees, I am orig from Ventura Co. and it was all over the place in the woods around there, San Bernardino isn't that huge a change in ecology. You will need a good ladder and a helper, be very careful, you are going to have climb the trees to get it.
You can find mistletoe growing high in the San Bernardino National Forest. It also grows on trees, especially conifers, in the San Bernardino Mountains. Talk to the forest service in your area about this for advice. Their contacts are available on www.fs.usda.gov/sbnf/.