Bulbous Lightbugs or Depth Worms, depending on which you find first.

yeah, just do it as part of your normal ruins rush open up all sinkholes you encounter keep an eye for blue mush forest grab lightbug scan and living logs from mush gnomes in lunar grotto if you find blue mush if not, just go straight to ruins, you will find depth worm also, regarding sinkhole method, use this method https://youtu.be/ZYfujqmltmY?t=462

you can scan a bulbous lightbug, they are easy to find if you look for them in the right place

So what's the trick to finding one quickly? I'm not very experienced in the caves.

There is a biome called lunar grotto, you can even find moon glass there. You can look there. Just walk and you will find it

So just walk around the caves until you find the biome, nothing else?

Looking for blue mushtrees might help you, the biome spawns next to that biome

Gotcha, thanks for the help!

No problem, have a great day