There's a post from 2015 similar to this one but it didn't provide me much help.
Here's the situation: I bought this card (GTX 960 SSC) off of a secondhand merchant on Amazon and they gave me just that- ONLY the card, in its anti-static sleeve. I thought that was all I needed, but when me and my friend tried to put it into my computer, we found that the only 8-pin power cable extending from my Power Supply Unit doesn't match up with the GTX 960, the pattern of holes on it are different than that of on the GTX 960.
To describe the pattern of the holes, it goes like this: (the Geforce GTX 960 words are facing upright) 
square-rounded-rounded-rounded
rounded-rounded-square-square
 
I have yet to find any kind of PCIE cable that might fit that. I don't know if the GTX 960 is supposed to have its own unique cable, or if I did something wrong with the 8-pin from my PSU and that one is actually all I need, or what. 
I've had this graphics card collecting dust for a couple months because it seems like nobody knows how to fix this. For the love of all things good, please someone end this XD
 
 
My computer is a Cybertron Borg-Q  with an MSI AM3 V X16 2D3 M16 GL R MATX motherboard and an LSP 550W ATX power supply.
---I KNOW MY POWER SUPPLY ISN'T VERY GOOD BUT PLEASE DON'T TELL ME TO REPLACE IT, I am not really interested in replacing it because, after all of this GPU stuff, I'm not in the mood for even more of a hassle with computer stuff---
Thanks in advance.

+1
 
Sound like a CPU 8-pin - from your PSU
 
You need a PCIe - GPU 8-pin (6+2) OR the adapter wire - though most will not use these. 
 
Your PSU must have enough amperage on the +12V to power the GPU - most would only use the adapter for testing in a pinch
 
As you can see below, the adapter requires Two 6-pin PCIe connectors on your PSU