You can just make your own sphere and invert the normals…would take maybe 5 seconds in Blender, for example.
Not quite sure what you mean…if you can see it, that’s the way the normal is facing. (Unless you’re using a 2-sided shader, but that’s rare.) If you look at a plane from the “wrong” side, it will be invisible, because the normal is facing the other way, and also objects will pass through from that side. Sort of like a one-way door.