You can buy chainmail armour from villagers and you can also kill zombie with chainmail armour on them but its a rare drop so don't get excited thinking you will find one. You can also find chainmail armour in the creative inventory. With mods and TMI (Too Many Items) you can craft them the same way as other armors but with "Fire" instead. That's cheating, though. :)
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iron diamond gold and leather
There is no way to make zombies wear leather armor, but sometimes zombies can naturally spawn already wearing the armor.
Helmet, chest plate, leggings, and boots.
Leather can be used for armor: helmet, chestplate, leggings and boots. Check out the link under "Sources and related links" below this answer for information about how to make armor.
weakest to strongest: leather iron gold diamond im not sure which ones stronger,iron or gold