Here you go, 8.4v rear. http://www.magicshineuk.co.uk/mj-818-rear-light-head.html
What gets me about those is there are no rear red lights that take 7.4v in. If I'm going to buy a light powered by an external battery I'd rather charge that one battery and have it power both lights.
It serves them right for travelling in the wrong direction ;)
They look a bit average to me
Perfect for blinding oncoming motorists (poo)