Staedtler Graphite 777 Eraser Refill?

I love stationery goods from Staedtler and Graphite 777 is one of my many favorites. It's easy to grip, works well, looks good - half transparent body with matching color eraser on top that, lasts forever and on top of all this, cheap. However, one of my concerns is that no matter how cheap it is, I don't want to buy a new one as long as it lasts and I'm almost using up the eraser. It is pretty big compared to the other erasers on other mechanical pencils and it erases pretty well too. I really want to buy refill for this one (gray) and can't find a website or store that sells refill for this mechanical pencil. Anyone knows where to purchase???