I always wondered why people wanted locking gas caps these days.
Aside from some specialized truck tanks, modern vehicles are very difficult to impossible to siphon. Trust me, I've tried to get mower or snow blower gas from my own cars. So a locking cap isn't necessary to prevent fuel theft. I've had fuel stolen from airplanes, but they have drains at the bottom of the tank.
Do these people piss off so many people, or live in such a bad neighborhood, that they worry about foreign substances being put in? If my neighborhood was that bad, I'd skip the new car, drive a beater, and move to a nicer place! Heck, even in the Bronx, people aren't randomly souring fuel tanks! They might strip or steal the car, but the cap wouldn't help that! 8^)
To the OP, any auto parts store can sell you a locking gas cap. You might have to ask for it. Bring the old cap with you.