Depends on the club.
For example, if someone's 'regular' club is MBOT in San Francisco, the answer to your question "How readily available are extras?" is going to be, "ROFL, you've got to be KIDDING right?" because they're so available that the club is FAMOUS for them.
Also, it depends on the city, and in specific, the sub-jurisdiction within each metropolitan area. For example, in the greater Tampa Bay area, the strip-clubs in Pinellas County (the area where Clearwater and St. Petersburg are, in the "thumb" that cuts the Bay off from the Gulf of Mexico) there's really not much going on, and the clubs themselves aren't even topless-bottomless (technically). In Hillsborough County (the "root" of the tumb, and the main Tampa area) there are tons of lingerie modeling shops. Many of these are full-service, and are de-facto brothels (and you can get more information ELSEWHERE on the internet, about the specifics) but, in some neighborhoods, the lingerie studios are actually ONLY lingerie modeling studios. So, even within Tampa, you have to know to find the Drew Park neighborhood, for instance. And even then, if you find the strip clubs within the Drew Park neighborhood, you're STILL not likely to find full service regularly.
Then again, things may have changed in the five years or so, since I've been there.
Hmm. Need to go back and visit again. Maybe it's time to plan a Thanksgiving road trip ... :)