Pachirisu Pokémon GO is one of the few Pokémon in the game that are region exclusive. Here's how to catch a Pachirisu and a few ways to get one if you're not in the right area.
Pachirisu is exclusive to Alaska and Northern Canada. If you're in these regions, you'll have no problem finding a Pachirisu in the wild. If not, you'll have to trade for one or wait for an event.
Pachirisu Pokémon GO: How to Catch
Previously Pachirisu appeared in Prague during an event, so it's not unheard of for regional Pokémon to appear outside of their normal range. If you know someone who has one from a previous event, you can always trade for it. Pokémon GO does have a location restriction for trading, so you won't be able to contact a kind Canadian or Alaskan to trade you one from afar.
Unfortunately, there's no official way to get a Pachirisu outside of the Northernmost regions of North America right now. If you want to go about a more dubious method, you can try a VPN to spoof your GPS location to catch or trade.
It is a fan favorite Pokémon, especially for those who were into the competitive scene back in the XY era. Pachirisu in Pokémon GO is a somewhat mediocre Electric type that only has any real use in the Great League. The electric squirrel will be relegated to your collection here.