There are many mastectomy boutiques and specialty shops that carry all types of prostheses and post-mastectomy garments. Most specialty shops employ certified fitters who are specially-trained to fit women for breast prostheses. Many mastectomy shops are owned by women who have had breast cancer themselves.
At your first fitting appointment, remember to wear a garment that fits properly (possibly a knit top), so that you can see the shape of your breast when trying on your new prosthesis.
Your physician, oncology nurse, local American Cancer Society Reach for Recovery program, or other breast cancer organization can recommend specialty shops and boutiques in your area that carry prosthetic devices and garments.