You should only need to call your GYN to get a mammo. I have to do that now as they aren't able to have standing diagnostic mammos.
My mammos are automatically sent to my GYN and my Onc, and they both look at them, or at least look at what the radiologist says.
I don't think anyone can walk into any facility and get an xray, which is really what a mammo is, as far as I know. There has to be an order written. This is insurance, not a doctor's choice. Otherwise, it would be chaos.