A good place to farm for swirlies is the wendigo cave south of the Desert Kingdom. The wendigos themselves don't drop a ton of swirlies but they often drop items and weapons that can be sold to Merchant Princess for a good chunk of change. Just make sure to destroy the skull after killing them. They'll respawn every time you go through the cave to get to Clocktown and back, at least
in the secret area of the cave.