Unfortunately, the correct cleaning rod for the Sharps-Borchardt model 1878 musket is a very rare item.  There are plenty of these rifles without the correct rod, but I have never, ever, seen a spare rod without its rifle.  The rod is unique to this gun, and has a sort of tulip shaped head with knurled ring at the top, and a round hole.  You often find these rifles with a replacement rod from a rolling-block or a Springfield.
But, don't give up hope.  A lot of guys are buying the military muskets and using the actions to make target guns.  If you run into someone doing this, hopefully they'll save the original rod.