Originally Posted by Grasschopper

Well SRAM makes a 12-26 which obviously isn't exactly what you are looking for but is a tooth closer than 12-25. I wonder if it is because 12-27 is used on a lot of cyclocross bikes and it is that time of year, plus Shimano might not make as many if it is primarily a cyclocross part. Just a thought, I was looking for them too a couple months ago and they are hard to get.