If your PSU doesn't have the connectors you need, that's usually an indication you need a better one. The recommended wattage for a GTX 960 is at least 20 amps on the 12v rail and 400+watts total. If you don't have substantially more amps than that on your 12v rail, get a different PSU.
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html