Find Nether Wart in Nether Fortresses, usually growing on soul sand near stairways and corridors
Loot Nether Wart from chests in Bastion Remnants
Trade with Piglins by giving them gold ingots for a chance to receive Nether Wart
Grow Nether Wart by planting it on soul sand or soul soil and waiting for it to mature
Use Silk Touch to harvest Nether Wart blocks if you need the block form instead of the item