Thank you for asking this question that will benefit a lot of readers. I can imagine that you would be worried about a lump over your chest.
An ultrasound is usually ordered by the GP or specialists to tell us more information about a lump. For me, I am a family physician, usually we will write a letter that will allow you to be scanned in a neighborhood location. Usually costs about a hundred.
There are some GP clinics that have an ultrasound machine. But I'm not very sure of these clinics locations at the moment.
Usually the doctor has to agree that a scan is required. We generally do not like to recommend unnecessary scans.