That sounds like a really non-standard setup. You could be in trouble. You'll probably end up with a solution that gives you the ATX connector plug and then take the various wires you need from that to connect to the PC.
There's nothing 'just' about that small part. That's what this entire forum is about. It's far from trivial to get those separate pieces.So I dont need the whole shabang, just that small part, which is why I have been having problems finding anything on the forum for just that.
Are you sure your PC doesn't also need 3.3V?