You're correct; EVGA does not give enough 8+8 pin cables. I ordered 2 extra from them (I think it was $20 shipped for two).

​

Then I order these for their 6 pins to convert to 8+8: https://www.amazon.com/gp/product/B079SS63WV/ref=oh_aui_detailpage_o05_s01?ie=UTF8&psc=1

​

Then for their 6+8 cables, I had to buy these to convert to 8+8: https://www.amazon.com/gp/product/B01DV1Z32Y/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

​

Most risers are powered by molex and not PCIE; I think there was one or two powered by PCIE.