RE: Derringer pistol, looking for a pocket gun.
If you're willing to go with a 22mag, take a look at the mini-revolvers from NAA or find a used one from Freedom Arms. They're both much smaller than the derringer and are five-shot revolvers. If you're wanting more punch than the 22mag, I"ll second what wolf and highpower said, Kel-tech, Kahr and Beretta all make a fine small auto.