Where did servants get crocodile dung for Cleopatra?

Well, the source of crocodile dung is a crocodile, so that's where it came from. There were/are crocodiles all along the Nile so it would be a simple matter for the slaves to collect the dung. However, it was not just for Cleopatra. Crocodile dung was an ingredient in many concoctions, both cosmetic and medicinal, in the ancient world.