You can gain Black Dragon Chest from every monster
You can buy Black Dragon Key from [NPC]Keymaker Keyro
You need to hunting on fast spawn n a lot of monsters spot, such as:
Moo moo's Inventory
Crow Spot
Clement Mine
Gun Spot
Sealed Cave
Sealed Island
Chaos Gear Spot
High Hamariel Spot (Forest of Cross)
Catacomb
2F Dungeon of Death
Northen Poibus
Souther Poibus
Etc
The best reward from Black Dragon Chest, such as:
Black Dragon's Guardian Glove
Powerful Dragon Claw
Bright Dragon Claw
Harmonious Dragon Claw
Good Luck