In Pokemon Y, you can get a Sun Stones different ways.
- Tierno will have given you a Peculiar Stone which you can take to Shalour City and trade witha hiker.
- In a Pokeball along Route 13
- There is a 5% chance it will be on a Solrock
- And in the Super Training Game "Drag Down Hydreigon" where you must obtain either a Deino, Zweilous, or Hydreigon Balloon. You might also get Moon Stone from this
Sources: Experience, and this page and this page.