I have visited Jonathan Browning's recreated shop in Nauvoo, Illinois, and looked at a couple of harmonica rifles he made. They're a really neat piece of work. I marveled at the forging and fitting of the parts; everything had to be close to perfect. I understand Sam Houston of Texas fame owned one. It would be a really cool thing to build, much less shoot!
You'd have to train yourself to use your support hand in front of the trigger guard like many military target shooters do even today in a standing position, using your thumb, forefinger and middle finger to hold the rifle. That would keep it behind the breechblock.
It doesn't look like they are being produced commercially for sale. It would certainly be a fun project for an enterprising machinist to take on though. I would guess there wouldn't be much of a commercial market for them, and the price point for anybody to make money at them would be too high.
A safer (monetary) bet if you really want one is to buy an original. That will be worth close to what you paid for it. A one-off basement special would be a long hard sale and probably not sell for more than a few hundred bucks. I saw a revolving flintlock made that way sell (on Gunrunner Auctions) a few years ago for only $450, and you have to know the guy had close to 1000 hours in to making it.