Search E-bay. "Iver Johnson shotgun parts" Try Numrich. Talk with a local crotchety Olde Gunsmith.
There are a lot of those old shotguns.Many have gone to parts.
Why not find one,make one,or have one made? Fix it!!
Firing any broken firearm isn't safe.
A great deal depends on which IJ shotgun it is too.
Gunparts is showing a brand new Locking Bolt Spring for an IJ Champion at $13.75. https://www.gunpartscorp.com/products/470770
E-Bay funds the people who want to take your firearms away from you.