Mine obsidian with a diamond pickaxe or netherite pickaxe
Find lava and pour water over it to turn it into obsidian
Use a bucket to collect lava and place water nearby
Break a ruined portal or Nether portal frame with a diamond or netherite pickaxe
Trade with piglins for obsidian
Find obsidian in chests in structures like ruined portals, bastions, or end cities
Use commands in Creative mode to give yourself obsidian