The Joker's Last Laugh (1) - S2-E47
Revealing mistake: The counterfeit 100 dollar bills that Gordon and O'Hara say look "so authentic" don't actually resemble any known U.S. currency at all. In fact, a close look reveals them to be very bad photocopies of Mexican 100 peso notes. (00:01:00)
Answer: There wasn't one singular city used. It was shot in and around various locations in California, plus on a lot of sets and backlots. Ex. Some scenes were shot in LA, some in Pasadena, some in Santa Barbara, etc.
TedStixon