Hardrock at Skem or Bridge St. Stone in Nelson might be able to help. See the Stone Suppliers page for their contact details. There are some imported setts/cubes coming in to Liverpool docks, but I've not seen them, yet, and I'm not sure of the price. I'll email you when I know more.
The actual number of cubes required will depend on how they are laid - if you use a 12-15mm joint, then the number of cubes per square metre is reduced to 81 (9x9, rather than 10x10)