Look for an LED UV torch/flashlight with a wavelength of 400nm. You could also use a diffraction grating, prism, or spectroscope, but you'll need to make sure

Look for an LED UV torch/flashlight with a wavelength of 400nm. You could also use a diffraction grating, prism, or spectroscope, but you'll need to make sure the light source going into it actually contains violet light. i.e. if you use one of those with a CFL or LED bulb, you'll most likely not get much (if any) actual violet.