Santa Clara Paintball on Wednesdays has decent turnout of around 6-10? only airball field. Im not so sure about how their friday/weekend turnouts are though. Other than that the only other places that has air ball fields would be around an hourish away from the eastbay
If you're looking for speedball, there's Fairfield, Midway, and Davis. If you want really good competition, Capital Edge in Sacramento is run by Dave Bains and a bunch of divisional teams practice out there.
Near the east bay your options are scp in santa Clara or Antioch paintball, I've never been to Antioch. But santa Clara Sunday seems like there's at least some people playing. If you can drive to the sac area, there's a lot more speedball.