These have been around in one form or another for quite a while, before i got the Karl Nill grip for my Ar15 this was something I had considered, especially as I like a more vertical grip like the Magpul K2.
I like wood so I'd put a nice rosewood on, but you could get all sorts of G10 grips and such for a nice grip.
Before MilArm went out of business I'm pretty sure they had an older on there (it was solid and heavy as balls) but I'm sure you could get one from Ebay or Amazon with free shipping or something. It's not exactly a highly complex part to make