I have no idea where you can buy these from as an aftermarket part, but when I worked in the headlight industry the best thing to use to cover the hole is a 'Gore-tex' patch.
Basically its a little nylon disc with goretex on the inside, stick it over the hole in your lights and these allow your headlights to breath without getting water in, but allow moist air out. I tried testing them in a variety of environmental test chambers and they work brilliantly.
You can also use a labyrinthe design which is found on some lights with a little rubber tube that fits onto a boss on the light and inside you put what looks like plastic moss. Same principle as the goretex. You could probably get these from a breakers yard, in an old japanese car, perhaps an older datsun or nissan.