[xattt]

Joining the thread to see where iron balls can be sourced.

Some vendor on AliExpress? Those Lucky Iron Fish?

[dgacmu]

This might work with old iron dumbbells (or modern iron kettle balls if left unpainted). Your gym might hate you for it though.

[lstodd]

I'd guess exact form wouldn't matter. Wrap a hammer in foil and beat up something rusty.

[mhb]

Cast iron pan left outside for a day or two.