Not too many derringer shoots in NCOWS, everybody seem to like the little pocket pistols better. And many places that have PP's in their matches call for four shots, so that puts a muzzle loader at a disadvantage.
Now personally, I like derringers. And I have a bunch of them, starting with a repro Colt single-shot .22 short. But my favorite is an original Remington double-barrel in .41 rimfire. Ammo is available from Navy Arms. It's a mite expensive, but rates really high on the "cool" scale.
For many years I used an American Derringer in .44 Magnum (!) which was allowed, even though it had the stupid cross-bolt safety. I also have a Uberti derringer in .357 Magnum, but they are really rare.
I guess what I am trying to say is that there are derringers available out there, but sometimes you have to look for them.
Grizzle Bear