Where did the shang kings obtain the slaves that provided them with one of their most important sources of revenue?

The Shang King's most important source of revenue was from Slaves that they were able to get as part of 'War Loot'.

At the time, there was a general custom that whoever won a war would get to take home a large 'Loot' which consisted of money, gold, ornaments and even slaves.

Shang Kings delonged to the Ancient Chinese Shang dynasty that existed almost 3,000 years ago. They are the oldest known Chinese dynasty with recorded history and archaeological proof.