We found them for £7 on ebay a while ago, but the postage would probably have upped them to more than the amazon ones.
We dodged the postage by buying a 12" at the same time from the same seller - it had free postage and the others were posted with it, so we just dodged postage costs completely, hence not being entirely sure on what it would have cost.
So yeah, I'd check ebay just incase, but more than likely amazon is the best you'll find
We had to pay £13.50 in person for one, wasn't impressed but didn't want to give Muffin one of our spare wheels at first when we could avoid it.