In California...the best you can get is 91 Octane.
I know of one place in San Diego that sell 100+ octane... but you can't pump it directly into a car. And it's not very close to where I work/live, so it's inconvenient for me. Fortunately, one of the tracks I go to sells all sorts of racing fuel. It's not cheap ($7.75/gal for 110 octane) ... but it's convenient!
GO by the local motorcycle dealership and put in about 3 gallons of race gas in before a fill up and you should be fine with the octane. The boosters only put deposits on your plugs, and increase the octane differently causing the plugs to go to crap.
I used to buy race gas at a motorcycle shop, but all they carried was leaded fuel, and it was about $45 for a 5-gal can. If anyone goes that route, make sure you don't get leaded fuel if you still have cats on the car.
And I agree on the caution about octane boosters. The only one that seems to be OK is the Torco concentrate (which really isn't a booster anyway).