In your area, I have no idea. Here in Pasadena/Los Angeles La Loma Development does beautiful natural swimming pools. Marco is an awesome guy.

Technically, a natural swimming pool is not difficult, so if you can't find a local natural pool builder, you could contract with a conventional pool builder and you tell them what to build. But you are going to have to be on top of it and tell them EXACTLY what to do.

I've had a natural swimming pond for years. They're very much a personal thing. What suits you may not suit the next person, even living just down the road from you. There's a lot of considerations to take into account, and afterwards a lot of tweaking. But I grew up swimming in dams and creeks so "natural" is natural for me.