If it doesn't come with all necessary cables (that's what I read in your post), and the shop where you bought doesn't sell the additional material needed, why not return it and buy something that is complete?
I just bought the ASUS Radeon HD7970 DirectCUII, but i can't get it to work...
I have a OCZ ModXStream Pro 700W Power Supply, with only one 8pin - 8pin PCI express power cable.
I bought 2x 8pin male to male cables on ebay, which theoretically should work, but they don't.
I've tried everything, but only the original cable that i got with the Power Supply lights up in green, the cable i bought from ebay lights up in red, i noticed that on one side of the original cable, there are 2 of the small cables connected to one pin. So i've tried to do the same thing with the new cables i bought (risky), and it didn't work.
I'm totally out of ideas right now.
I asked in every computer store in the area nearby and nobody has a cable like this.
I thought to buy a new power supply, but im not sure which one would be the best for this video card.
Motherboard: Sabertooth x79
Processor: Intel Core i7-3930K @ 3.20GHz
Memory: Corsair Vengeance 4x8GB DDR3 RAM
Power: OCZ ModXStream Pro 700W