The Detective’s Office & Barber Shop (10246) includes the first mirror in a LEGO set. It was released 1st January 2015.
According to New Elementary, the piece is the same size as 1x4x6 windows, it comes in a bag with stickers to protect the sides from scratches and the part ID is 20193.
Alternatively, there are also highly polished bricks available in chrome and gold that reflect more light then others. These bricks tend to be rare and expensive.
There is a non-purist solution that works well using metalic paper or tin foil which can be found in craft stores and supermarkets respectively. The material is relatively cheap, flexible, can be as small or large as you like and provides a decent reflective surface. You can use double sided tape to stick the material onto slopes or you can hold them in place using bricks.