Following the eShop closure in March 2023, the only way to get is via archival sources. The most reliable, verified source currently is the hShop (the homebrew eShop replacement). They host clean, verified dumps of every 3DS update, sorted by region and Title ID.
If you are looking for a specific or installation guide for a custom firmware setup, let me know: Are you using a standard 3DS or an emulator like Citra? Do you need help with FBI installation steps? pokemon x update 15 cia top