When you got your psu it should have come with cables to fit every slot, and good psu's will often include extras with different connectors. Your problem is going to be that its probably not a PCIe male connection on the psu. Most modular psu's use proprietary connections, even as far as the same brand eg. You can't use corsair AX cables on a corsair CX. Your solutions are either get lucky and find one on eBay that is used by your exact psu, or contact the brand manufacturer and buy one from them. If all else fails you could also use a Molex to PCIe adapter, but I would only do that as an absolute last resort.