The ones I have seen come with the battery. Have a look at ebay item number 120723180985 and see if that's what you are after.

 

No he means a case (Silicone, Leather, etc.) not the battery cover, your right the spare battery covers usually come with the battery door.

I strongly doubt that there will be anything on the market suiting what you want funkytwig but I would suggest one of the generic leather or cotton pouches, I understand it's not ideal but it may be the best you can do, personally I don't use a case at all, just a screen protector and I'm pretty happy with mine's condition, got a couple of scratches but it's already outlasted most of my other phones.