You can add alternate power. There are supplementary PSUs for such things (without the mainboard connector) or you could just use another PSU (I have done that before when adding hard-drives caused my main PSU to shut down due to drawing too much power to tide me over until I got a more capable main PSU). I have also used a 12V supply for a IDE/SATA to USB adapter to power an internal hard-drive in a pinch.
This is slightly risky in that if your voltages are slightly off, you could theoretically end up having too much current flow where it should not but that has never been an issue for me.
Here is one pulled at random from google. Not recommending, no association, etc.
http://www.cputopia.com/fsp-group-booster-x5.html?utm_source=googlepepla&utm_medium=adwords&id=20206522684&utm_content=pla&gclid=CKuR--TJxLICFQQ4nAodgkIA_w