There are no official links provided by Ubisoft for downloading Assassin's Creed IV: Black Flag NSP or DLC for the Nintendo Switch. The game can be purchased through the Nintendo eShop, which is the official digital storefront for the Nintendo Switch.
The Switch version integrates various resource packs and cosmetics that were originally sold separately: assassins creed iv black flag switch nsp dlc link
If you are a developer, a hobbyist exploring your own legally obtained digital backups, or you fully understand the risks, the general process for installing an NSP file on a CFW Switch is as follows: There are no official links provided by Ubisoft