If your Switch is not modded, just:
If you don’t own it, buy it from the eShop.
No NSP files needed.
If you are manually installing the Lost Castle Switch NSP update, you might encounter these issues:
If you own the game on the eShop, you can create a legal backup only if your Switch is homebrew-enabled—but that process voids your warranty and risks a ban from online services.
While this article does not host or link to any NSP files, it is critical to understand the risks if you go down that path: