Lava Bucket
minecraft:lava_bucket
Made using a Bucket rather than crafted directly. Doesn't stack.
How to get it
No direct recipe — it's made using a Bucket:
Crafting 
→ 
Bucket
Villagers buy this
Armorer · Journeyman 1 → 
1 Emerald +20 XP
1 1 Emerald +20 XP