Yes. The supply of breast prostheses for women who have had a mastectomy is usually organised by the local specialist breast care nurse.
There are a number of different manufacturers of breast prostheses. At least one provides a prosthesis that has been tinted to match black skins, and another will tint a prosthesis to match particular skin tones (though this may take some weeks). These are available, free of charge on the NHS.
Your mother should talk to her specialist Breast Care Nurse, who should be able to give her all the details.
You can also contact Breast Cancer Care who offer advice about prostheses.