I doubt if you'll find anyone offering granite shelved racks as an online product. It's just to difficult to ship granite safely and cost effectively. I made my own audio rack (with the help of an expert carpenter friend) and had the granite shelves custom made at a local granite kitchen and bathroom countertop business. I supported the granite slabs on a sheet of mdf to give it lateral strength to support heavy components without breaking. Otherwise granite will not work as a shelf. It's just too brittle.
You can easily make a granite shelved audio rack by ordering a aluminum audio rack kit made to your dimensions. I don't know the company but Rampage60 can provide you with the contact info. See Rampage60's amp stand in System #101. After assembling the frame just order the granite of your choice from a local granite counter business. However, some of these businesses aren't real interested in very small granite orders but don't mind selling their remnant pieces.
If you like solid walnut you may want to check here. I like the looks, design and price of the audio furniture from this company. They're located in SanFrancisco but they sell online and often advertise on Audiogon.com. I especially like the amp stands with the isolated shelves.
http://www.corefurniture.com/