You didn't say where in Northern California, thats a pretty big place, bigger than some eastern states.
I would look in the local phone book and on Allison's website. There are so few of them, you will probably want to use the closest one.
In Portland, Or there were only 4 within 100 miles, and Allison directed me to one 20 miles away for Warranty work (2 were less than 9 miles away).
2002 35R Southwind, W22 8.1L Vortec UltraPower, 19.5' wheels
Toad: Wrangler, lifted and on 35" tires