I ripped the little mirrors from a discoball, but I'm sure you can buy them at some hobbystore somewhere
Or else.. Just find a discobal you can pull apart ^_^
Make sure you can somehow remove the mirrors from it and that they are seperate from each other.

This project costed me about 6 euros: 3 for the glue & 3 for the discoball
I already had the bra..

If you're planning to make one yourself.. be prepared to get your fingertips/nails covered in glue