Your PSU should have the required cables, if not you'll need to buy a new PSU.
this is my psu https://www.amazon.com/Corsair-Bronze-Certified-Modular-CP-9020103-NA/dp/B01B72W1VA/ref=sr_1_1?ie=UTF8&qid=1525377387&sr=8-1&keywords=corsair+cx650
That PSU will work fine, it has the required 6+2 (8pin) connections needed to power the GPU.