Hello!
To be honest, there is no way to be certain on whether or not we will get a release for these shiny Pokémon. However, it is likely, as most Mythical/Legendary Pokémon have their shinies made public at some point via an event. As stated, there is no guarantee on this, but it has a high probability of occurring.
To answer the second part of the question, Serebii and other websites get the information of shinies thanks to datamining. When someone goes into the game's files to find info, every Pokémon's regular and shiny sprite can be found. This allows places such as Serebii and even this website to display every Pokémon's shiny form, even if its not released at the time.
I hope I helped, and feel free to let me know if you would like me to turn this into a comment, as it doesn't give a concrete answer to the first part of your question.
