Where Do You Get Shroomite Bars

Shroomite Bars are a crafting material, crafted at an Autohammer with Chlorophyte Bars and Glowing Mushrooms. The Autohammer can only be acquired after Plantera is defeated.

How many Shroomite bars are in a full set?

A full set with all three headpieces requires 78 Shroomite Bars (1170 Glowing Mushrooms and 390 / 468 Chlorophyte Ores).

How do I get the auto hammer?

The Autohammer is a Crafting Station that is purchased from the Truffle for 1 Platinum Coin after defeating Plantera. It works just like every other crafting station: the player stands near the station and scrolls through a list of what they would like to create.