It is difficult to find even 10 gauge shotgun ammo today. I have my doubts if 8 gauge shotgun ammo is even made any more, but corrections welcomed.
There is a ammo clip for a J C Higgins shotgun model 58324 16 gauge. You can buy it as a replacement part.
Standard 16 gauge shotgun shells.
go to an ammo shop
$4 or so.
I do not know of any ammo makers that still load ANY round ball shotgun loads. However, there ARE 28 gauge slugs (Brenneke makes them) that are available. An internet search using the term "28 gauge slug ammo" will show several companies that carry them. They are about $11/ 5 shells plus shipping cost, so they are not cheap.
Generally speaking, you have to be 18 to buy rifle or shotgun ammo, and 21 to buy handgun ammo.
Do a web search on "12 gauge ammunition types" Over a dozen.
12 gauge shells.
I like buck shot. OO.
How 'bout a slug or buckshot.
Buy it