Do any concave slopes or curves exist? Any brand Do you know if anyone has ever made any concave slope pieces? Looking for multiple lengths (2/3/4/5/6 stud lengths). Basically the opposite of this style: 
I would also be looking for the same concave shapes, which would go underneath, similar to this: 
The first example is a graduated curve, the bottom seems to be more of a constant curve. I would prefer graduated if possible, but in the concave direction, with the shallower slope at the bottom, steepening towards the top.
If not, does anyone know anybody that makes custom shapes or pieces?
Part 18653 Brick, Arch 1 x 3 x 2 Inverted

Brick, Arch 1 x 5 x 4 Inverted

And a few other inverted arches