http://www.allpinouts.org/index.php/PCI_Express_(PCIe)_8pin_powerMay be able to splice out 3 12-v+ lines and the grounds from molex and splice in to a 6+2 or 8 pin PCI-e. Scavenging from a dead supply would be the requirement here...
Or spend like 5 bucks and wait...
http://www.amazon.com/s?ie=UTF8&index=blended&field-keywords=molex%20pci-e%20adapter&tag=fstchrm-20