[ZeoRangerV]

The Daleks were initially a humanoid race (the Kaleds) before their chief scientist began manipulating their form into what they are today. The Kaleds were already embroiled in massive nuclear war that was beginning to take it's toll on them, but Davros was trying to persuade that the Daleks were the only way the race would survive. Of course, Davros was also a megalomaniac who wanted to be certain his creations would go on to become the dominant species in the universe

Before they eventually turned on Davros (he wasn't a Dalek, therefore he wasn't fit to lead them), there was already an assembly line for breeding new mutants and building the cases. As they've improved things over time, odds are they've used slaves to put the lines together and fix them whenever it's needed. Daleks are always taking new slaves.