I'm not sure which prepbook companies put out review books just for one subject. But don't fret too much, the reason for the dearth of ochem review materials may be the fact that ochem is only 30 percent of the Biosci section-- it's just not worth stressing over. Whatever prepbook you have right now, the ochem that's covered in there should be plenty. As proof: I was a straight-C ochem student, but got a 12 on my biosci mcat.