Find and enter an Ocean Monument
Defeat the Elder Guardian to obtain a Wet Sponge room reward
Break Wet Sponges found in the monument
Kill Elder Guardians for a chance to get Wet Sponges
Use a bucket to collect water from a Sponge room area and explore the monument thoroughly
Smelt Wet Sponges in a furnace, blast furnace, or smoker to dry them into Sponges
Find Sponges in some generated Ocean Monument rooms
Get Sponges from creative mode inventory
Use commands to give yourself Sponges in worlds with cheats enabled