how to spawn mechanical bosses without hallowed bars?

[FreshlyFishedBread]

so fargos mutant requires me to kill every boss again to get their summoning items in his shop but i have no hallowed bars to craft the summoning items.

[Foncik]

There is a 1/10 (10%) chance that one will randomly spawn each dusk after the first Demon/Crimson Altar is destroyed in a world, and each will continue to spawn randomly until it is defeated at least once. They also drop from all Hardmode enemies (except statue-spawned enemies, Meteor Heads, and Old One's Army enemies) with a 1/2500 (0.04%) chance until their respective boss is defeated. Says wikipedia page.