Your local dealer (cringe) should have them - ask for door lock knobs from a late model SLK. They can't be more than a couple bucks each and are the narrow head type (not the mushroom head type) and retract into the doors when locked. Enough so that the knob doesn't get trapped, but can't be pulled up with your fingers. Works well for childproofing the rear doors.
They're a bit tough to remove from Euro cars once installed - I think they use Lock-tite on the threads.