CNC, is a form of computer controlled machining, normally used to make new parts. To use it to port a head, they would have to 3d scan the existing heads, then on the computer. map out what needs to be cut. I would bet the only place you will find such a service, is a nation wide speed shop. And then, it might be only for the high volume engines, like a chebby small block.
The only advantage a CNC job would have, is they could set it up once, and then let the mill do many heads, all the same. To do one set of heads, I would imagine it would be super expensive. Likely several hundred dollars just to have the computer operator scan, and set up the program.
Porting and polishing heads, and intakes, is amung the simplest modifications to do to an engine, and can be done by yourself quite easily. I would not spend the money to have a shop do it.
CNC, is one of those fancy buzz words, like billet, Sounds high tech, but in the end, means very little to the end user/ consumer.