In the main powdercoating thread there are a couple bay area suggestions. One for East Bay and one for South SF.
Two people suggested Maas Brothers in Livermore (I asked them about pricing and they said "We usually charge $90-$135 each wheel. If the wheels are powder coated from the factory there is an additional $25 chemical stripping fee for each wheel.")
One person suggested Precision Powder Coating in Belmont