Peter
They are all around you. To start with, Oregon white oak, Quercus garryana, second cousin to California's Valley Oak, Q. lobata, are terrific for bonsai, leaves reduce, vigorous growth, yellow fall color. They have typical lobed oak shaped leaves. If you want to go dwarf, there is Q. dumosa, scrub oak. Full size leaves are under an 1 1/2 and they reduce even more. Very slow growth, small internodes, but they are evergreen. Canyon Live Oak, Q. chrysolepis make terrific buttressed trunks even as shohin, evergreen. Coast Live Oak, Q. agrifolia are fine in your area as are all of these down to 15F (but they are a host to Sudden Oak Death syndrome SOD. I don't think this is a consideration for bonsai as long as you are not near the coast. There is also Cork Oak, terrific bark, leaves reduce, evergreen, also good to 15F. I have grown all of these and I am almost exactly the same climate as you. There are even more species native to the Northwest that would work, do a little reserach for some more dwarfs. I know there are a couple more dwarfs that grow in the Sierras. The only disappointing one is Black Oak, Q. kelloggii; it has wonderful fall bright orange yellow color, pink buds in spring, but the huge leaves do not reduce. Compared to Eastern Oaks, you are in Oak Heaven.
Brent
EvergreenGardenworks.com