You will find very few as random drops in barrels. not in chests.
But the trader on the ship in Cxseal harbour and the 2 sellers for assorted alchemy items (like needles and dolls) in Cyseal and in Silverglen start to have fiery hearts in stock after you venture out to Luculla Forest, so about level 10.
I always found and bought enough for at least 6 of them.
Regards,
Thorsten
