Travel to the Nether
Find glowstone clusters hanging from the ceiling
Mine glowstone with a tool enchanted with Silk Touch to collect the block
Mine glowstone without Silk Touch to collect glowstone dust
Craft glowstone blocks from 4 glowstone dust in a 2×2 crafting grid
Trade with cleric villagers for glowstone dust
Find glowstone in some Bastion Remnant loot chests
Use commands in Creative mode or with cheats enabled: `/give @p glowstone`