Iver Johnson 12 ga Ring Trigger Ammo

I have an old 12 ga IJ Ring Trigger.. It has cleaned up beautiful. The bore is a tiny bit pitted but overall quite good and shines on very well.

Does anyone know where can I get the proper ammo to shoot this little baby? I have read it absolutely will not fire modern 12 ga shells. Thanks

I also posted this in "Ask the Experts" forum as well.