Where can I find a shiny stone, because I want to evolve my Roselia?

Route 8: Sees it right above the scientist when you first enter.
Lake of Outrage (Wild Area section, west of Hammerlocke. Requires the ability to cross water): Look around the circle of stones. Finds every evolution stone there. Respawns eventually.
Digging Duo (Wild Area): Takes some luck. Should be possible from both siblings. Cannot say which is better, though.

Edit: Also as a "Great" reward from 7-star and 8-star Poke Jobs.