First is it not a tree, but a parasite that grows on trees.
You can find it on the following trees in CA.
alder
flowering pear
ash
birch
box elder
cottonwood
locust
silver maple
walnut
zelkova
Oaks
Alder
apple
black locust
maple
white fir
It is easy to spot and very easy to harvest. You generally will desire female mistletoe over male, as the female variety produces berries while the male only produces polen. Just look for large green clumps of vegitation in the trees. Removing it is good for the trees by the way.
Or you can just order some via
http://www.mistletoegift.com/