Free: Contests & Raffles.
I would start with the NMLRA. They would be the best resource that I know on who to contact regarding UH locks and barrel. Also try UNDERHAMMERS.COM. They have a UH locks for sale on AuctionArms website, Item # 107329008. That should get you started.
Fire and ironhttp://www.fire-iron.biz/Will have the locks, as will Blue Grousehttp://bluegrouseblackpowder.webexone.com/login.asp?loc=&link=You can buy a percussion barrel from Green Mountain Barrels.
Give Blue Grouse a call.
Contact me if you have questions on shooting a underhammer. They fire every time if loose clothing doesnt get in the way of the hammer. If you don't want to build one I have rifle from Blue Grouse for sale if you want. .54 cal with a 1/32 twist. It is a tack driver.
http://www.longrifles-pr.com/actions.shtml
I like the cantilevered scope mount for the peep because on my wife's gun, with the peep not attached to the barrel, the sights can get turned when cleaning as there is no rotation lock between the action and the barrel. The only problem with mine is that I don't think he used a laser bore sighter when marking and tapping the second screw as I have to have a scope adjusted all the way to one side to hit the paper with it.He does respond to phone calls better than email.