I think he's missing the whole lock, not just the key.
Keys can be made easily by any locksmith for Cessnas.
And +1 on joining the Cessna Pilot's Association. I'd rename it the Cessna Technical Information Association personally... not much info on flying or flying technique, but their sources and resources for maintenance are the best out there...
A quick phone call to them, they'll not only tell you where to get that part, but the part number, and at least one other alternate way to lock up your airplane.
They highly recommend the addition of locks that you cut out a slot in the bottom of the window frame and mount them just under the window outside the aircraft door.
The reason? Grab almost any Cessna single with even a slightly older door hinge and pin assembly way out at the wingtip and shake the crap out of it...
The door WILL pop open, even if locked, just from the twisting of the airframe.
Now remember what all the Cessnas were doing the last time the wind came up on the ramp... the rock-and-roll...
Yup. Doors will open on their own, unattended, in a frisky wind.