Radio Shack used to sell them... I looked on their site - no luck...
One other thing to keep in mind, if you can find an AC powered unit with a display that will fit (and it's cheap enough that you don't mind losing the warranty), check the part number of the bridge rectifier, and voltage regulators in it.
I have a VU meter that was AC powered, but the bridge rectifier and voltage regulators dropped the voltage from AC 120 to DC 13. I just popped a couple wires in PAST the bridge/regulators, cut the AC cord out, and use it in my old car. You might be able to do the same thing with a clock and just drop the shell and mount the display in the 3½ face plate.
GL!