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
Glue them - one by one - on your bra. One tiny drop of glue is enough for one mirror, just don't use too much or it'll get messy!
(make sure you put them exactly right the first time, because they won't come off easy once they're glued on)
I started at the edges, and then worked my way into the centre :)