45-60 was one of the calibers developed for the 1876 Winchester along with 45-75 and 50-95. The latter two calibers used a necked cartridge to achieve the larger charge.
I have no idea where you could find a dummy round made up though.
Uberti reproduces the 1876 in 45-60 and at one time several of the places that load cowboy action ammo were selling both the brass and loaded rounds. Buffalo Arms still sells both loaded rounds and the brass. The loaded ammo is expensive at $61 for 20 rounds plus shipping, but the brass is priced by the piece at $1.50 each. They also sell cast bullets, but not in small quantities.
Here's a link:
I would suggest calling them during the day and tell them what you are trying to accomplish. They might sell you a couple of pieces of brass along with the appropriate lead, so you could make up your own dummies.
However, it's worth mentioning that one can fabricate the brass by simply trimming down standard 45-70 brass. The 45-60 is simply 0.16 inches shorter version of 45-70 brass. If you reload or know somebody who does and they have a case trimmer, the following video by Mike Beliveau will show one what to do.
Get a couple of pieces of 45-70 brass, trim them down and glue in the appropriate size bullet. Probably a 300 grain 45 cal round point flat nose will work.
Alternatively, if you know anybody who shoots 45-70's, get a couple of loaded rounds from them. Carefully extract a bullet so it's not deformed, empty out the powder, shorten the case by .16 inches, detonate the primer with a punch, and force the bullet back in the new shortened empty case.
I hope some of this proves helpful