Man, searching for Ash and Shauna stuff can feel like digging through a pile of old trading cards sometimes. You'll find some real gems buried in the usual spots, but the 'quality' part is so subjective.
For me, a solid chunk of my favorites are on Archive of Our Own, obviously. The tagging system means you can filter for their dynamic specifically—'Ash Ketchum/Shauna (Pokémon)'—and then sort by kudos or comments. That usually surfaces the popular, well-written ones. I've stumbled on a few writers there who absolutely nail their competitive-yet-supportive energy from the anime specials.
Don't sleep on fanfiction.net either, even if it feels older. You have to sift more because the tags aren't as precise, but some of the classic, longer-form adventure fics for that pairing are still hosted there. I remember one called 'Viridian Dreams' that was a surprisingly tender slow-burn, set after their journeys.
Honestly, though, some of the best snippets and headcanons I've seen are scattered across Tumblr. It's less about full narratives and more about character studies and dynamic posts, which can be just as rewarding if you're into their potential. Following the #pokeani tag often leads you right to it.