Diancie at the moment can only be obtained by event distributions. The first one was in Japan from July 19 to September 30, 2014 when the movie was first released. Diancie was soon obtainable in the US on October 27 to November 16, 2014 and it was obtained by inputting a serial code in the Mystery Gift option in the home menu. The codes were always random and they were given out in Gamestop. (The cards are the ones shown on the image below!)! Now a days, Diancie isnt that hard to find since a lot of people cloned it when it was first released in Japan, so a lot of people do have multiple Diancies.
A shiny Diancie is impossible to obtain normally because its shiny locked. However, due to Power Saves existing and the QR exploit, people are now able to obtain a shiny Diancie without fail.
If you really want to obtain a Shiny Diancie, you may want to look here for a tutorial on how to do so on IGN: Click here. However do it at your own risk since its rumored that this can ruin your game but I'm not sure. You may also want to look for the QR code for Diancie. You can use this one here but it may be a little blurry.
Hope this helps!
Sources: Bulbapedia, Serebii, and IGN