The short answer to your question, as mentioned, is there were no military made left handed guns. Even today there are no present day left handed guns made that I know of.
I am left handed and I shoot mostly right handed guns. Most of the time I shoot them left handed with no ill effects. You will find the issue of the cheek piece a non issue just as I have.
The only concern in that respect would be a more expensive fowler or rifle with the cast off made for a right handed shooter. Cast off is a slight bend in the butt stock. More simply put, the butt plate is off set to the right compared to the center line of the barrel. For a right handed shooter it makes sight alignment easier and causes the stock to move away from the cheek on recoil. Shooting a gun with severe cast off from the wrong side will result in a slapped cheek. Unless you find a nice custom really cheap this should not be an issue.
Most rifles and smooth bores available in the entry level market are strait stocked. The same can be said for kits. By entry level I mean a good used custom or semi custom or a newly made semi production or production gun like TVM or Pedersoli.
Another thing to consider is learning to shoot right handed. I can shoot right handed and accurately too. In most cases I shoot left but I can shoot right which really helps in hunting situations. I'm sure that skill would have come in handy fighting Indians too.
The Pedersoli offerings like the Bess, 1777 French, US 1803 rifle and US 1816 muskets are available new in the box. But because prices have risen for the European guns in the past few years a Custom made Bess or even a rifle is money well spent.
search,
Roy Stroh
Mike Brooks
Sitting Fox
North Star West
Track of the Wolf
classifieds here
flintlocks.com
There are many others but this should get you started.