If your psu doesnt have enough power cables for your GPU,that means it was never meant to power such GPU,and dont think about using adapter,they are fire hazards and most of the time dont even transfer enough wattage to the GPU,which can cause the GPU to fry itself.
That unit is not good by any means.I had a similar "10$ 500W psu" with my r9 380,and quess what,it didnt work.
It is strongly recommended that you never cheap out on a powersupply since its the power source of your system,and you want clean power to be transfered to your components.What is your budget for a new one?