Try a fitness equipment store. My doctor wrote a prescription for 2 pieces and that meant I did not have to pay tax on them. I was in a horrible auto-pedestrian accident and spent months in the hospital and rehab facility. My doctor wanted me to keep using some key pieces that helped me so much in my recovery so that's why he wrote the prescriptions.
That's probably TMI
but maybe if you decide to get a large piece, tell your doctor about it and see if he thinks it's beneficial to your health and he might possibly be able to write a prescription for you. Good luck.