Compared to the Mokoko Seeds that number over a thousand, the Giant's Hearts collectibles number at a much more reasonable 15. This means that each one is a feat to collect, however. Be prepared to spend a significant amount of time, effort, and/or resources for each Giant's Heart you find.
You can trade for the Giant's Heart in Blackfang's Den, but that doesn't make it easy to get. You'll need to fork over a tidy sum of valuable currency to claim this heart for your own.
How to get the fifth Giant's Heart in Lost Ark
You can trade for the fifth Giant's Heart with Mariner Rosa. You'll need to offer 3,300 Gienah's Coins.
You can get Gienah's Coins by opening up reward chests that you get for completing various quests on islands. Alternatively, there are daily activities that you can do to potentially earn more Gienah's coins.
Adventure Islands spawn every day. These islands have the potential to offer more coin chests, which you can use for more Gienah's Coins. You can see what time the islands spawn and what rewards they offer by clicking the compass icon underneath your minimap.
For information on where the eighth Giant's Heart is, go check out Where to find the eighth Giant's Heart in Freedom Isle in Lost Ark? on Pro Game Guides!