That's right ($35 for most folks, $10 for people with very young kids), they're doing walk-ins for the pertussis booster right now (http://sfcdcp.org/pertussisiz.html). Everyone under 1 or with a kid under 1 (including pregnant women) should get the shot ASAP.
How did it die in the first place? Is there conclusive evidence that it was the vaccination and not just due to a cumulative effect of better hygiene, better diet and better living conditions? And when and how did it resurrect itself? These questions need to be answered, because if there was an incorrect assumption in any of the previous answers, the current solution may not be as good as we'd hope...
Your GP can give it to you, or if you happen to be in San Francisco there's a clinic at 101 Grove Street that will give you the shot for just $10.