The little fiddly bits on costumes can be a pain to find when you don't know the correct term for them.
Plastic Cabochons, for example, are the foil-backed acrylic gems found on 90% of anime characters. I've never seen anything but the faceted variety in my local craft stores while most designs call for the smooth ones.
Acrylic Half Spheres pretty much the same as above except colorless and no backing. These would be good if you needed a custom color or a massive size (up to 2", anyway).