Meteorite Bar

Smelting your own from a fallen meteorite is the obvious route, but bars turn up in chests too. Shadow Chests in the Underworld carry them regularly, and the deep gold chests that generate just above hell hold them occasionally.

A lucky chest at the bottom of a hellevator can hand you the Space Gun and the matching armor set before you have killed a world evil boss, which is well ahead of where that gear normally arrives. The set makes the gun cost no mana at all, so a run that stumbles into enough bars early gets a functionally infinite weapon several hours ahead of schedule.

It is rare enough that you should not plan around it, but it is a good reason to open every gold chest you pass on the way down rather than skipping the ones that look like they only hold potions.