If you are talking about just Alolan Pokemon or Alolan forms then I'll help you out. I'll put some Pokemon I know can be found in-game but are not from Alola for e.g Magnemite can be found in game during themain story!
Pichu can be caught really early and you can evolve it into Alolan Raichu which can counter Water Types.
Grubbin which evolves into Vikavolt, if you want a counter right now, then don't get Grubbin because the evolution from Charjabug to Vikavolt will happen very late into the story
There are more but that are some suggestions.