In the game you are playing, there is no such thing as a ‘sun shard’. You cannot use an item to evolve Eevee into Espeon in Alpha Sapphire.
Eevee evolves into Espeon primarily through a hidden mechanic referred to as both ‘happiness’ and ‘friendship’. It is a number assigned to each of your Pokémon ranging from 0 to 255, and is a means of evolution for many Pokémon, including Eevee.
Eevee will evolve into Espeon if you play during the day, and if you level it up while its happiness is above the number 220. A list of ways you can increase Eevee’s happiness can be accessed on Serebii here. For general play, it would be easiest to equip it with a Soothe Bell and level it up a lot.
If you’re curious how close you are, travel to Verdanturf Town and talk to the girl pictured here. You can roughly tell what number your Pokémon is up to based on what she tells you; a guide is featured here under ‘Happiness Checker’.