The wee aluminium plates that go under the cleats on your spd shoes, my pedals are starting to mark the carbon soles.

does anyone know where I can get them from?

Ta da: Crank Bros Shoe Shields

CRC do them as well.

The Crank Brothers shoe shields work well.
If you’re going to use them, best to fit them as soon as possible.
I found that once grooves had worn in the sole of the shoes, the shoe shields would bend slightly in to the existing groove.
Starting with the flat surface of a new shoe, this wasn’t a problem.
I think if I was ever going to fit shoe shields to an old pair of shoes again, I would fill the grooves with Milliput or similar first.