Left hand stocks are readily available from other suppliers, just gotta shop around until you find what you want.
These suppliers can provide about anything you want.
http://www.longrifles-pr.com/
http://www.muzzleloaderbuilderssupply.com/
http://www.dunlapwoodcrafts.com/
http://gunstocksplus.com/index.html
http://www.gunstockwood.com/
IMHO, I would save the really nice wood for a future project, since first time builders often ruin their first stock, so IMHO, a plain stock is less expensive to make into firewood, than an expensive full curl stock.
IMHO, it is fairly safe to fire a right hand flint gun left handed, so I wouldn't worry about that. IMHO, if you have a stock, you can do almost anything with
it...within the parameters of your skills and the amount of wood on the stock blank.
IMHO, there is nothing that says that a longrifle
must have a cheek piece, so you can rasp off the existing cheekpiece and make a left handed stock out of it.
I have an original Missouri made half stock with no cheekpiece that has a full profile on the left side of the buttstock, with a slightly flattened area above the centerline that forms a kind of very subtle cheeckrest, so you might have enough wood on your precarve to form a very subtle cheek rest on the right side of the buttstock. If not, then so be it.
The first thing I would do is pick up a coupla books on gunguilding. Alexanders "Gunsmith of Grenville County is pretty good, as is Shumways "Recreating the American Longrifle".
A couple of videos wouldn't hurt either.
You can rent several videos from Smartflix less expensively than buying them, the downside is that you won't have them for reference once you get into your build. However, since renting is cheaper than buying, you can rent several and then buy the ones you like best.
http://smartflix.com/store/category/75/Kits-Scratchbuilt
And since you need to know what something looks like before you can build it, I suggest buying or at least reading as many books on antique rifles as you can get your hands on. Most of the good books are available through inter-library loan, if you don't want to buy them.
Looking at a few web pages, of originals and very good contemporary pieces is a poor second to having the books at hand, but at least there are decent photos at these sites.
http://americanhistoricservices.com/html/home.html
http://kindigrifles.com/ and click on the "available" tab to see some nice originals.
Eric Kettenburg's web page has lots and lots of photos of originals and extremely well done contemporary pieces.
http://web.mac.com/kettenburgs/Site/Home.html
The contemporary blog has lots of nice contemporary work.
http://contemporarymakers.blogspot.com/
God bless