Semi-relevant business first:
Fangraphs is having a meetup tomorrow night at 21st Amendment.
McCoven in general can organize NLDS meetups here.
*-I will not give you free beer.
And don't fuckin' drink and drive. Drunk driving makes Sullivan cry. Seriously, though.
So unfortunately, Southern Tier does not distribute to California. Which is a damn shame, because their Pumkingis by far the best pumpkin beer that I've found (and the rest of their stuff is pretty fuckin' good, too). They are in a good chunk of the rest of the country (including Oregon), however, so if you have the chance, grab some before it's all gone. And drink it quick; it doesn't age very well.
As for the Bay Area, there aren't a ton of local options, though I know 21A and Magnolia make some, and I know Almanac is working on one to release in the future. One of the best available to us, however, is Midnight Sun's TREAT, which is an imperial chocolate pumpkin porter, which tastes exactly like all of those very good words in a row ought to. It's pretty hard to find, though, so you may have to do some digging (or ask your friendly neighborhood beer store to order it). I randomly found it at The Page last year, which is something I'll have to test again in the very near future.
Elysian Brewing also hosts its Great Pumpkin Beer Festival every October, so if you're in Seattle, you should probably just ignore everything I'm saying here and go do that. Around the Bay, though, Elysian is scarce.
Also, bewarned: Jack's Pumpkin Spice is a Michelob beer in disguise.
The best combination of quality-to-accessibility, though? Probably Shipyard Brewing Co's offerings: Smashed Pumpkin and Pumpkinhead. Personally, I like Pumpkinhead better, but I am literally the only person I know who shares that opinion, so go ahead and get yourself a bomber of Smashed Pumpkin wherever fine beer is sold.
Oh, and here's a relevant tasting write-up by my friend Brian Yaeger: http://blogs.sfweekly.com/foodie/2010/10/pumpkin_beers_tasting_blind.php