Kill spiders
Kill cave spiders
Break cobwebs with a sword or shears
Fish it up
Trade with a fletcher in some versions
Find it in dungeon chests
Find it in desert temple chests
Find it in jungle temple chests
Find it in bastion chests
Find it in ruined portal chests
Find it in shipwreck chests
Find it in village chests
Find it in woodland mansion chests
Find it in stronghold chests
Find it in mineshaft chests
Find it in ancient city chests