Installing a USB 3.0 card into an Optiplex 755 SFF

Hello, I want to buy a USB 3.0 card to fit into the PCIe card slot on my computer however I can't seem to find where I'd get the power for it, the external hard drive I'd be plugging in would require power so I need it to have power.

I'm at a loss for what to do, can anyone help me, I'm a complete novice with this stuff.