It's powered by the PSU. The whole point of having powered risers is to stop the motherboard burning up trying to supply to all those GPUs.
Some people argue that they shouldn't be necessary because the load on a PCI-e slot is defined in the specs, so boards should be up to that spec, and cards should not pull over that spec. Do you want to experiment to see if it's true?
So which slot/port do I put the molex cable in the psu? Do i need to buy cables? I have a corsiar rm750. Should I put it in pheriphals?