Hawkin type guns have the replaceable barrels over ramrod and one screw into butt of stock. Got that on some of my T/C's. Otherwise, the leather strap around barrel and pouch for butt plate style of sling. However, these get used rarely! Gun is usually cradled under arm. Only truely good for long walks when not expecting to see game!
I bought a real nice home made one at the gun show at the Wash,Co.Gun Show a couple weeks back.A real nice older couple have a shop in Beaver Falls called Shablesky Muzzle Loading Supplies.The Mrs.handles the m/loading stuff and makes the slings herself and other leather products.It was only $15.00.Her number is 724-843-5925.
Not sure where I got it but I use one like these with my Lyman Great Plains. It is suede, laces onto the butt stock and loop over the barrel with care not to get into my site picture. I don't use it all the time or even every year. If I'm out for a long walk in the state gamelands, it is nice to carry it slung.
I've been hunting locally the last few years, so the carry in to my stand is short, just carry where the gun balances as someone else has said.
In the past on my other muzzleloaders I have used a slip on shotgun sling that seemed to work pretty well. Its a modern sling and doesn't fit the look of the gun but it works and that's all I care about.
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.