Hi Angela. I'm Cindy. The Dollar Store sells small square mirrors. You should measure the door in order to know how many mirrors you need. Glue them to the door using E6000 glue (also from Dollar Tree). Once the mirrors are all in place, you can frame it with trim from your local home improvement store. Hope this helps. Good luck Angela.
You can just buy a mirror and use the mirror corners to attached it to the door.
Depending on the look you are going for, Dollar tree has lots of little mirror squares as Cindy suggested, I have also seen them with patterns in them. Or you could measure and get an uninterrupted look from Amazon/Ikea. How you attach it would depend on the size and weight of the mirror.