Silver ones almost always drop them. You gotta fight those ones. There's a coliseum in the Depths where you fight 5 Lynels in a row, two of which are silver. I get 2-3 guts when I fight them after each blood moon.
Now what was really rare to find was normal, green Lizalfos Tails, because I would only see the blue ones or higher. It was tough to find green ones late in the game, and the tails drop so rarely. It was worse than finding Lynel guts.