It took me a while to find how to obtain the official mappings, but this article seems to have instructions: https://minescript.net/mappings
According to the article, official mappings can be found here: https://piston-meta.mojang.com/mc/game/version_manifest_v2.j...
They're also linked on the wiki page for each release, along with links to the client and server jars: https://minecraft.wiki/w/Java_Edition_1.21.5