Smogon University - Pokemon Showdown! has been updated with new HD sprites. Please have a look->

OUTDATED] The Beginner's Guide to Pokémon Showdown

Suggestions for Pokémon Showdown, Page 69

Sticky - Smogon Sprite Project

will they update with sword and shield sprites this time? : r/ pokemonshowdown

A Week or so in the Shoes of a Sprite Leader - Smogon University

Programming - Sprite Converter

Spriting Incognito – A Guide to Disguises - Smogon University

Pokemon Showdown: The Underground Community of Smogon University – New Media: Storytelling in Literature, Films, and Games

Spriting Update (Guide: Pixel Overs) - Smogon University

How to sign up for Pokemon Showdown - Quora