You can fit any knob with M12x1.25 thread by using reverse lock adapter.
I have this on mine: http://www.ebay.com/itm/BLACK-LIFT-UP-REVERSE-LOCK-OUT-SHIFT-KNOB-ADAPTER-FOR-MANUAL-SHIFTER-12X1-25-/200845204381?hash=item2ec34e9b9d:g:1tMAAOSwyLlXn7oK&vxp=mtr
And this knob: http://www.ebay.com/itm/M8-M10-M12-Heavy-Weighted-Round-Ball-Shaped-JDM-Black-Shifter-Lever-Shift-Knob-/361384890208?hash=item542437cf60:g:dioAAOSwBiJZcDcC&vxp=mtr
Here's a pic how it looks: