So I looked on Ebay, and found that the best deal I could get was for some produced by Avon Tyres, not a mile from where I'm moored, in Melksham. I cycled past the old RAF base there a while back, and saw that the hangars were full of tyres, and great racks of them on the tarmac where once the roar of the Merlin would have been heard).
But I couldn't just go and buy them over the counter (or perimeter fence). So Jan in Holt kindly took delivery of them, and I cycled over and picked them up. This morning I started putting them in place.
For the low hitching point, the rope that came with the fender was too long; so I spliced the surplus onto the one destined for a higher hitch.
Two done, four to go.