Waterproof arm cast. Can anyone tell me where I can get a waterproof arm cast fitted, or where I can buy one?

My son has a broken arm, and has a normal cast on it. I have read that you can get a waterproof cast, enabling you to swim, bathe as normal. The product is GORE� PROCEL� Cast Liner,but I can only find it in America. Does anyone knowhow I can get one fitted in the uk, or where I can buy a similar product to get it fitted.