Are you saying that your power supply is a modular one and you can't find the pci cable that was supposed to come with the power supply. Either I am reading your post wrong or something but you said that the card has two 6 pin inputs and your power supply has two 6 pin outputs what is the problem then.
The 8 pin you have is the end split into a 6+2 connector or a solid 8 pin. It's hard to believe that the PS doesn't have a 6 pin. Take a close look at the end and see if it is possibly a splitable connector.
If your PSU has two 6 pin PCI power connectors and your video card needs one 6 and one 8 pin connector, just use both 6 pin PSU connectors. If you look really close at an 8 pin (or 6+2) PCI connector, you will see that the two extra wires are ground lines.
Assuming the same wire gauge, an 8 pin connector cannot carry more power over its 3 hot wires (the yellow ones) than the 6 pin connector can carry over its three hot wires.