Mirror mirror on a stick

I will be trying to show the Venus transit to a group of school pupils by
using half of a pair of binoculars and reflecting the light path (down to a
sheet of paper in an open topped cardboard box) by using a mirror on a
stick. Does anyone know if/where I can get a plain surface silvered mirror
of approximately 50 mm diameter?

Any suggestions will be gratefully received.

TIA

Peter