I just troll the Bazaar for all the Sunstones and Sandstones (For the Transmute Recipe) I can get. That's pretty much the "fastest" way there is for Sunstone.
Other methods include:
Gardening (But that is only a chance at getting it from each harvest in selected plants)
Farming Bosses and creatures that drop it.
Farming Sandstones around many places like Savannah, Stone Town, Floating Mountains etc.
