I have found a site called 'knitted knockers'. they will knit you a cotton breast prosthesis for free. Sounds odd, but they are much more comfortable, and lighter, than the silicone ones. They even have knitting patterns for them on their site, so you can 'do it yourself'! Hope this helps.
Am sure your mum is eligible for a free one. When I had mine fitted I was told I could have a replacement one every couple of years. Contact your local Breast Cancer Nurses - am sure you can find out which hospital by phoning this helpline.
I am not sure on this, but a suggestion try ringing your local MacMillian centre they might be able to advise you
I'm actually posting on behalf of my lovely Mum. She had a mastectomy in 1978 ( she's done brilliantly, and is a massive inspiration!). She's now almost 81 years old.She's always bought her own prosthesis, but now she's older and only got a pension coming in,I was wondering if there was any way she could claim any money back for what she pays out for a new one? She desperately needs a new one and they're about £140. Are there any forms we can fill in to send off with receipts after she buys one? Even if it's only a percentage-anything would help!