Find a shop that caters to Cowboy Action Shooting and you can get shells that are loaded with black powder. Because of the age I would hazard a bet that it cannot take a full charge modern load. You can also but mini-shells or low powder shells.
I have a similar era Ithaca 16ga and use the cowboy action shells in it without problem.
http://www.sassnet.com/
http://www.buffaloarms.com/
(I frequently go to Buffalo Arms for 45/75 bp ammo and reloading supplies and for all my other Cowboy Action needs)
If you are interested, I also have 12ga brass cases for reloading. Shoot me a note.