I saw one today on a Honda 400-4; it even said 'Honda' on the cover. It didn't seem very new, but if you go to Screwfix they sell dayglo trousers for about £11 and you could use the material to make a couple of covers. They even come with an elastic waist, which you'll probably need to hold the cover on to your headlight. The Screwfix trousers don't seem to have any retroreflective stripes, so there's more fabric to choose from than on some others. Search for 'Hivis'.