What year and kind of Mini do you have? I am leaning towards replace but it would help to know what Mini you have
Fog lights in the lower bumper or the ugly ones that look like Mickey Mouse ears mounted to the upper grill? Adding the year of MINI you have will help others help you.
My guess is the lens is not available as a separate piece. Rather than buy new I'd recommend car-part.com. I'm sure you can find a used one for not much.