Without making a custom replacement I would guess that they don't exist.
The light was tailored to fit the car, has to fit the hole in the roof perfectly and I don't suppose many aftermarket companys get much demand for them.
I don't know if higher models have interchangable ones, such as units that have two separate lights etc...........maybe somebody could clear this up.
I had a search around and it seems nobody makes an aftermarket light unit, looks like your stuck with the origional or you will have to make a custom one.