Been meaning to get around to something like this.
This particular one is not for sale, I borrowed one of my roomies D20's to do this so I can't sell it =P HOWEVER! If you have an interest in these please let me know! I'm thinking of selling them at cons and possible online as well! I'm thinking of finding cool D20's to do this with ('cool' being marbled or different kinds like the one shown).
EDIT: I just ordered some cool dice so keep an eye out for slings with dice for sale!