I'm with Fat Tom... I've seen two factory all-target 18's in over 40 years of collecting and those were both very early factory ordered guns when special orders were still possible.
There are sure to be others out there, perhaps owner modified guns, but you may have a long, long wait finding one that's original. I'm not sure if other gun parts in their frame size are interchangeable but if they are you may want to add the necessary parts from a 17 to a standard Mod 18. That would seem to be a much more achievable goal...
Just sayin'