OldMaine,
I've asked this question many a times on many a board and the answer has always been no. There may be kits out there, but I haven't found them. I spoke with a Sgt at Williamsburg once about it and he stated that he would have beaten any soldier who didn't shoulder his firelock to the right shoulder.
In those days left handedness was considered the work of the devil and was beatenout of children at an early age. My mother, even, was forced to learn to write right handed in grade school back in the 1920s, so it hasn't been very long that us sinister folk have been allowed to use our left hand for much.
You may have to roll your own or opt for something other than a bess. I've heard tell, on this board and elsewhere, that shooting a right-handed flinter isn't all that bad and the few times I've tried it really didn't bother me much. If you're concentrating on the target you tend to forget about the flash. The only problem I encountered shooting a rightie was when I had to use musket caps on a nipple designed for #11 caps - put a pretty good tattoo on my wrist.
If ya hear about a leftie Bess, be sure to post it here. I'd love to find one!!