Bought ours at Argos. Linky here. If you want one that just fits over 1 burner then most camping shops do a round one that fits the suitcase stoves. Around £7 or £8.
I know this is a griddle pan but wasn't sure if it was this or a plate you were after, it might be small enough, I think the stove is about 19" wide isn't it? (Chapman Catering are doing them for £4.25)
Katie
Had another look...........it might be a bit too heavy, I'd need three hands to lift it - back to the drawing board
Bought a cast iron double griddle from a local, household goods seconds store, cost Euro 5.00.
Fits well across the 2 burners of my Optimus #22 stove, but even better across 2 Optimus #00 stoves side by side.
Not really a backpacking item though ... bl**dy heavy!
Ikea have non-stick aluminium ones that look good.