Rollei T/Rolleicord V - magnifier

Does anyone know where I might get a lens insert to fit the flip-up magnifier on a Rollei T/Rolleicord V in the US, UK or Europe? I have one of each of these cameras, one magnifier is the right strength to view using the distance part of my bifocal glasses, the other one is too strong and gives a focus about halfway down the finder hood instead of on the screen. All info gratefully received!