There are a few sources:

Kill Milians and shred them, they have a chance to give a chip of their type. Fighting with the Milira or Skyfeathered Church will provide opportunities. Skyfeathered Church is particular gives you a lot of Milian quests, but if you are a Milira you probably need to wait for the Church to attack you.

If you are a part of the Skyfeathered Church you can unlock Milian drop permits. These lets you get the units and some basic gear for them without a chip. You could then disassemble them for the chip to use it to make a different type of Milian from the same class.