Mine iron ore with a stone pickaxe or better
Smelt iron ore in a furnace, blast furnace, or smoker with fuel to get iron ingots
Find iron ingots in village chests
Loot iron ingots from chests in structures like mineshafts, dungeons, strongholds, and bastions
Trade with armorer, toolsmith, or weaponsmith villagers
Kill iron golems for iron ingots and poppies
Find iron nuggets in chests and craft 9 iron nuggets into 1 iron ingot
Use a fortune-enchanted pickaxe to increase ore drops
Mine iron ore in caves, mountains, and underground layers
Use a silk touch pickaxe to collect iron ore blocks and smelt them later