Look for a Sunrace 8 speed 11-40 cassette and an 8 or 9 speed Shimano Altus or Alivio rear derailleur with a total capacity of 45 teeth. You can check the exact derailleur model name on the Shimano information portal.
The existing 8 speed shifters should work just fine with the new derailleur, but check to see if they're Shimano before you go spending any money. Also remember to get a new chain to go with the new cassette so the old elongated one doesn't skip over the new cassette teeth.