Got it off ebay. Type "diamond paste" into ebay. You'll get about 20 hits, but scroll right down to the bottom to the ebay stores bit. You will see two suppliers, Abtec and Mattools. I got mine from Abtec for £4.99 as they only charge a quid for postage.

It is probably not as good as a water based paste from an application point of view but I can't tell you what a massive difference it did to the sharpness of my razor. Normally my razor shaves OK off the 8K Japanese waterstone but pulls a bit in the thick places of beard. This time it was through it like silk. Unbelieveable.