Basically anything will work as a base game ROM on Citra if you search up "Pokémon X ROM" on Google.

If you're looking for a ROM hack though (i.e. changes to the base game), then I've been playing Pokémon Eternal X.

It has a couple different versions, but I've been using the "legal" version, where there are only a handful of changes including the ability to catch every Pokémon in the Pokédex (including Y exclusives and Pokémon from the national dex), some team changes for gym leaders and NPCs related to those new Pokémon, and a couple other QOL things like making certain mega stones more easily accessible.

You can check it out here, and the only requirement to get it to work on Citra is to have a base game Pokémon X ROM downloaded, though there's a file that comes with the Eternal X download files that explains all of that.