Capt55, One source of parts for this class of firearm is A.R.Matt..ask via an e-mail to firstname.lastname@example.org
If you mean the Davis D-32, not the O-32, here is a link to the schematic and owner's manual.
Cobra stocks parts for the Davis industries derringer, mod D-32, DM-22.. The part you need is C 404 -16Q standard derringer from Cobra. I just replaced my selector. Late message, just signed up.