Where can you get a slug barrel for 12 gauge JC Higgins Model 60 shotgun or at least smoothbore with no polychoke and vented rib?

The only way to reliably chamber shotgun slugs in your JC Higgins Model 60 shotgun is to get the barrel modified by a competent gunsmith. It is not uncommon for classic smoothbore shotguns like yours to have an obstructed bore due to the way they were manufactured - the polychoke and rib could get in the way of a slug as it's pushed down the barrel. A gunsmith can remove those features, smooth the bore, and thread it to mount a rifled choke tube that you can use to fire slugs and other specialty ammunition designed for rifled barrels.