You need to look at embellishments with adhesive on them, such as hot fix crystals. Here's a site I've ordered from before:
http://www.suessparklers.com/
There is a special tool that you can use that heats up the crystal and activates the glue, however, if you are just going to be doing a few, you can position the crystal and just use your iron to heat it up. One of those small clover irons works well