Hi Alice
I'm an osteopath in UK where I work manually with my patients.
However, the advice above to contact your physiotherapist would be the best course you could take.
Therapists of all varieties are sent catalogues by the dozen from suppliers of medical equipment and it is probable that your practitioner could recomment a particular unit for you to purchase. Your orthopeadic consultant is unlikely to have had a great deal of experience in the use of ultrasound machines as he would refer a patient to a physiotherapist.
Before spending a lot of money remember that professional operators of these machines undergo many hours of training.
It is very easy to buy a car but it takes a lot longer to learn to drive!

Colyn.