You can buy matte finish sealant at art supply stores. At craft stores that sell tole painting supplies (i.e. things for mom crafts) you'll be able to buy matte sealant for sure. It's often available in a water-based brush-on version. I only recommend the spray stuff if the brush-on won't work cause of solvent incompatibilities. (i.e. water based paints need a solvent-based coating or they melt like runny watercolour. If you used something solvent based to make the art (such as a sharpie or nailpolish) then I recommend using a water-based finishing spray so the paint underneath doesn't melt.)