“Are there IR mirrors available?”
Yes IR mirrors are available and can be used for many different purposes. They can be used where a direct line of sight is not available (to help see around a corner with a camera), and they can be used where the cameras is too close to the target and adjusting the distance is not possible.
Any material with a very high reflectivity can be used, and the easiest solution is to use a polished metal with a mirror finish. Gold is often used as it does not suffer from oxidisation. For temperature measurements, you will need to measure the reflectivity of the window. To do this you follow the same procedure as measuring transmission and use the reflecting surface in lieu of a transmitting material, and the end result will be reflectivity instead of transmissivity.