I know that in Pokemon X you can get Mega Stones after beating the game and some through the main story and Dianthus gives you a tali’s with Gardevoir, but where can you find Galladite?
Pokemon X and Y do not have access to the full set of Mega Stones - some were introduced in Pokemon Omega Ruby and Alpha Sapphire.
Galladite is one such stone: it was introduced in Pokémon Omega Ruby and Alpha Sapphire. It cannot be found in - or traded to - Pokemon X/Y.
Source: Bulbapedia - Galladite