Where to buy garden stone ball?

[MrChips]

Something like these stone (or stone effect) balls:






I've tried a few local garden centre type places but no joy!!

This is the current state of our front "garden" so am looking for a few bits to add into the pebbles. I've got a few plants already, and planning to either get some nice pots to go under the bay window, or to build something out of sleepers to hide the gas meter away a bit more. Other ideas very welcome!