I'm in Iziz with Mandalore, and I';ve questioned a few people about the missing droid's head that may coutain info about the imprisoned doctor, but I'm having trouble moving forward. I've taken the three captains out, but what do I do now? Do I have to find the droid's missing head, and if, so, any hints on where it might be?

You need to find the droid head. Head inside the cantina again and talk to the beastmaster/mercenary guy and the rest of the people then head outside to the droid seller has the head.