every computer i know of uses a 12V ATX cable (mostly 4-pin, but some use an 8-pin), and as far as i know, every ATX PSU comes with one. check and see if you didn't confuse it with a PCI-E connector (those should be 6-pin, but sometimes they'll be 8-pin).
if you're using a modular PSU, the 12V ATX connector should be on the non-detachable strands of wires from the back of the unit. and, no, it doesn't detach from the 24-pin main power connector (the part that detaches does so to fit older motherboards with a 20-pin connector).
anyways, let us know if that helps.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind