I found these ones, http://www.suremotorsports.com/products/performance/ags-shift-knobs/
and I quite like this too, http://www.bloxracing.com/bloxshop/index.php?route=product/product&keyword=knob&category_id=0&product_id=319
I'm not sure I like the round ball ones, they look a bit odd. I guess...