Like almost all other Shinies, you can't catch Shiny Eeveelutions, but you can catch plenty of Shiny Eevee. Once you have a Shiny Eevee, you can evolve it into any of the Shiny Eeveelutions. The nickname trick will work if you haven't used it already. Otherwise, you'll need to evolve them just like any other Eevee. The same goes for the flower crown costumed Eevee.