No Apple Store will offer this option. You likely will need to find a third party that sells the casing and you'll need to disassemble the current one and move everything over.
Other option is to just simply buy a skin of your choice and just install it over it unfortunately.
About the only custom option you're going to get from Apple is engraving - and I'm not sure they still offer that.
There used to be a third party vendor that would create custom colored casings and then swap your current phone's casing. The very thing @Apple_Tiger mentions. Not sure they still exist, but I imagine other vendors have since filled the gap. Haven't looked in quite a while.
I'm sorry, Apple is not Burger King - you take what they offer because Apple won't let you have it your way.