Im not sure how your wires terminate but I dont think you will find an adapter that will fit into the PC molex adapter.
Have a go at wiring it yourself, it will be good practice for you. Dont worry, you cant really do any harm with 12v from the PSU.
All you need to do is to hook the LED wires up to the yellow (12v) and black (ground) wires of the PC PSU. You could wire them up to a spare female molex plug and then plug that into the male molex from the PSU if you dont want to butcher the PSU.
Check out this [
coin door wiring video], it may give you a couple of pointers
Have a go