They sell them at Jamie's Italian - named the best olives in the world, I think. You don't get many, but they are probably the best I've tried in a restaurant.
They also offer them in Zizzi's and Carluccio's now - they aren't on the menu by the name above. I believe Zizzi's call them 'Goddess Olives'. Not an exaggeration!
I have bought some from a tiny deli in Brixton (where my brother lives, seemingly an unexpected wealth of foody experiences!) and they weren't too expensive. I ate them all in one sitting.
I've searched online and every offering of these beauts seems to be wildly expensive, with high shipping costs. If anyone reading this knows where I can get them, please do get in touch via Twitter (@Jenbo36). I will give you a mention on my widely-read, highly regarded blog.