First, let me welcome you to the Forum.
You honor us by taking the time to join and participate.
To answer your question about Underhammer locks, first let me say that although there were a few companies making guns with this style of lock about 20 years ago, most have gone out of business. Those remaining, no longer produce underhammer guns.
To my knowledge, there are no companies making guns with underhammer locks. A good gunsmith can create one, but it would be a custom item.
Note that I said "guns with underhammer locks" and not just locks.
This is because the reciever or frame of the gun is part of the lock for most underhammer designs.
The underhammer lock has just 2 moving parts. The hammer and the trigger.
The hammer piviots on a pin thru the reciever and has the sear notches machined into it.
The trigger has the sear machined into the forward end of it.
The Triggerguard serves the purpose of being the mainspring.
If you look at my sketch, I think you can see how these parts relate to one another:
This sketch is somewhat crude and there are several designs which have been used over the years. This sketch just shows the basic idea.
I hope this helps answer your question.