Unfortunately, I don't. I got a very nice piece from BRS but I don't think they sell them any more. They are a distributor for Marco Rocks, who used to be another source for good shelf rock, but they no longer sell directly to the public. Not sure if BRS can work with you to get you what you are looking for or not.
Premium Aquatics sells Real Reef rock and caries the shelf rock.. Email them and I am sure they can get you a good piece. I was buying it local for a while but it was cheaper from PA even shipped.
That stuff is fantastic for shelf's.
Here is a shot of a older tank with some of their shelf rock mounted to the back:
That middle peice is almost 2 feet wide.