Sunshine in a flower
Sundelions are extremely useful in cooking in The Legend of Zelda: Tears of the Kingdom. Drop a Sundelion into your recipes and it will remove gloom damage, freeing up some of your locked-out hearts. They’re also useful in some of the quests, like Gloom-Borne Illness and Mattison’s Independence. However, if you don’t know where to look they can be hard to find.
Where to look
Sundelions can be found on the surface, but they’re rather rare. Typically, you’ll want to look around fallen chunks of sky ruins, as they occasionally pop up there. This fact, in itself, offers a clue for where you should be looking for Sundelions. You need to look skyward.

If you search any of the sky islands, you’ll probably find Sundelions in abundant quantities. To be specific, if you launch yourself from the Skyview Tower in Eldin and land yourself on one of the floating platforms in the area there, you shouldn’t have to look too hard before you are tripping over them.
One other place you can find them is through Yiga Clan member drops. This is maybe not the most practical way of collecting them, but it’s worth noting. Whenever you take down a Yiga assassin, check to see if they dropped a flower alongside their normal bananas and rupees.
Since these aren’t found easily on the surface, it’s a good idea to scoop up all that you can find whenever you’re walking in the clouds. Not only are these useful in holding back the gloom, but they also pop up in standard quests. It will save a lot of time if you just make sure to line your pockets with them whenever they’re seen.