The black factory ones that I got are plastic.
I also got some after market ones from Chigs Grips (http://chigsgrips.com/?page_id=7).
The SP101 grip inserts/panels are the same dimensions as the GP100 compact grips but they are not the same size as the regular sized grip inserts.
There is also a difference on the inside of the panels between the GP100 compact and the SP101 in that the SP101 panels have a spacing hole bored out to make room for the peg in the mounting apparatus.
I don't know where to get the plastic black grips outside of ebay.