All five tiers of Exp. Candy (XS, S, M, L, XL) appear in this game. This is worth noting because XS candies were absent from Legends: Arceus.
The easiest method to acquire Exp. Candy before significant completion of the game is to farm them from Alpha Pokémon. Alphas drop Exp. Candies when defeated and respawn frequently. Higher level Alphas drop better candies at higher rates, including Exp. Candy XL according to one source I found. I've seen people try reloading these Zones to quickly respawn Alphas but I've tried this and honestly it seems more efficient to simply fly to each Zone sequentially and by the time you're done the others will have respawned and you can restart the cycle.
However, Exp. Candy S, M, and L can also be purchased directly from the Quasartico Inc. Mega Shard Exchange, depending on your progress through the game's story.
Exp. Candy S is available immediately after unlocking this shop, after completing Main Mission 10.
The other two become available after completing the main story. They cost 8, 30, and 100 Mega Shards respectively.
XS Candies have a high occurrence rate (20%) as hidden items throughout the game, especially on rooftops.
Source (Exp. Candy Location Data). The rest of my info is based on testing and research.
