A guide on farming Bokoblin Guts, which can drop from Bokoblins and are guaranteed from Silver Bokoblins.
About Bokoblin Guts
You can get Bokoblin Guts from Blue Bokoblins, Black Bokoblins, and Silver Bokoblins. Bokoblin Guts are guaranteed from Silver Bokoblins.
How to Farm Guaranteed Bokoblin Guts: Silver Bokoblins
Silver Bokoblins are found in the Depths beneath the Hebra Mountains, the Depths beneath Crenel Hills and Crenel Peak (east of Hyrule Castle on the Surface), and the Depths beneath the Gerudo Highlands. Defeat Silver Bokoblins in those three areas for a guaranteed source of Bokoblin Guts.
Where to Find Other Bokoblins
Other types of Bokoblins are more plentiful, but they aren't guaranteed to drop Bokoblin Guts. Black Bokoblins are commonly found in the Depths under southeast Hyrule, the Depths under southwest Hyrule, and the area of the Depths near where Tanagar Canyon is on the surface. Blue Bokoblins are most commonly found near roads on the Surface.