I have also considered getting a sling for my longrifle because even though slings aren't historically accurate for longrifles, my arms know it after a day of carrying around the 9 lb rifle. However, I haven't gone through with it yet because of the following...
If you have a longrifle, you would have to attach the sling either by installing a front swivel pin, which would not be historically accurate for a longrifle, or you can go with one of the universal slings that ties to or slips over the barrel and the buttstock or trigger guard. These are available from Track of The Wolf, the Leatherman, Crazy Crow, October Country, and probably other places. Before you buy one of those, keep in mind that the front tie will go somewhere between your front and rear sights. If your sights are low, there is a good chance the sling will obstruct your sight picture.
If you have a hawken style flintlock, you might have some more options. The sights on my Lyman Trade Rifle are much taller than those on my longrifle, so a universal sling might do better. I've even seen universal slings tied to the ramrod, but I don't know how well that would work, aside from the obvious hindrance of drawing your ramrod.