Hi Jennifer, purchasing "mirror" cut to that size would be very expensive. They do make mirror spray paints that I've been pretty impressed by. Its not a perfect mirror finish but it looks pretty neat. Spray painting the backside of the window will make it look more authentically like a mirror.
Jennifer....Depends on the window but you might be able to remove panes and find a mirror the correct size to add to the back. You will need to maybe add screws with a small metal plate that will hold mirror in.
Could you paint it with mirror paint?
Here is an idea for a repurposed window mirror https://www.pinterest.com/pin/633178028840937108/