My basic approach to hard battles etc. is to stock up on meals that add extra heart containers (and to close my eyes at the scary parts).
Up until recently, I've had a solid back stock of Durians, Hearty Truffles, and other things that add yellow hearts when you eat meals cooked with them. But I'm pretty much out now, and need a location or two that I can reliably find some more.
Best Answer
Unfortunately, finding reliable locations for specific foods is somewhat of a challenge. However, there are some general areas that seems to have a few good harvests.
Durians are the easiest to come by, in my opinion. If you go Faron Woods (the jungle-style part of the world, in the southern part of Hyrule near the Spring of Courage), many of the trees will have 2 or 3 durians hanging from their branches. There's at least one grove of about 6 trees, which will get you a whole bunch of durians. Here's the best way to find the specific spot:
Hearty radishes are found randomly in "normal" biome areas of the map (non-snowy, non-desert, and non-volcano), so there's no place to harvest them specifically. That said, every blood moon I warp to Skull Lake in Akkala and head due east to fight the nearby Hinox and Lynel. Around that area are two hearty radishes that seem to respawn fairly reliably.
Truffles seem to be just as randomly generated as heary radishes. Since they spawn mostly at the base of trees, going to a heavily forested area should yield a few more of these. There's also a pair of NPCs near Hetano village who are searching for truffles - I haven't looked for anything nearby, but there's probably some around that area as well.
You can also buy food items. At the Korok mushroom store (in the Deku Tree), the clerk will usually have at least one truffle for sale, usually 2 or 3. At Gerudo Village you will find a fruit vendor who sells 3 durians. And Beedle sells hearty lizards at at least one of this stable locations, though I can't remember which one exactly.
And don't forget, you can also take a picture of any of these items with your Sheikah Slate+ and track them with the radar. It's a little tedious, but it does work very well.
As a final, tangentially related note, if you've got a high amount of base health, these "hearty" ingredients can be extremely efficient for making full health food. Because you need to be at full health to have temporary hearts, using even a single ingredient in a recipe will completely heal you. This means if you find 10 truffles/radishes/durians/hearty lizards/bass, you can make 10 full heal potions.