The source we have available about vikings comes from the Celts.
Step-by-step explanation:
Celts are the settlers from Scandinavia who scattered all over the Western Europe and islands beyond it and settled those lands.
The Vikings were their neighbors in many of their settlements and there were few Celtic cultures like the ones in Denmark that were close to them and had some genetic intermingling.
This resulted in the documentation of the viking ways by the Celts from whom it has been passed down to us.
These documents include sagas like the Viking Saga and history of their kings.