Most local print shops seem to have, or know someone close by that has access to a cnc mill. You may want to ask around your town first that way you can save on shipping costs, customs, and such
I was surprised that the Mom & Pop sign shop I buy materials from actually had a huge CNC in the back. They were willing to make anything I needed from them, but unfortunately the guy that they hired to operate the machine was "still learning" and I didn't want to be locked into paying hourly for something that shouldn't take more than a day or two to make.
If you do find one that can help you just have a set of questions for them like what kinds of file types do they prefer with their software, hourly rates, and try to tactfully ask to see work that they have done just to make sure it's within their scope. I've seen some charge a flat quote for a job (typically the very experienced that know they can deliver) and others that charge hourly - I've seen $20/hr up to $60/hr so be prepared to REALLY want something made that you can't build on your own
You may also be able to save some money if you have the model ready to go for the type of machine you want it cut from. So if you are having it milled on a 3 axis, make adjustments to the model accordingly and make the cleanup/prep process as easy as possible for whoever is milling it for you. Having a breakdown sheet printed off with screenshots/renders of the model with dimensions is also helpful.
A quick note on the file types - a lot of the "sign" shops may not have the software needed to properly handle hi-res 3D models since a lot of these types of services cater to making plaques, headstones, and uh signs
So if they look at you like you're crazy or are hesitant about what you want don't be afraid to look elsewhere and save everyone time.
-Hugh