Michaels Craft store, or any craft store, but you can also get them in the nail isle of any drugstore. You can get them in diff colors and they can come with adhesive already on them (which would peel of your face easy) or you can get them without the adhesive and just use a bit of hair gel to stick them on your face.
I'd go to Sally's or some other beauty supply store and pick up the rhinestones used for nail designs cuz they're pretty small. Eyelash adhesive would work well too, I think.