In my story, one of my characters is sent into a cave by a modern-day shaman to get a certain ingredient for a healing potion. It needs to be something that isn't immediately available online for order. His wife is dying, so he isn't going to want to leave her side, but if it means saving her life, he will. I already have the scene made out where he is going into a cave to get the ingredient, what I'm having a hard time with is picking the ingredient he is getting. All the mushrooms I've read about in caves are readily available to order online. The snakes that live in caves aren't venomous, so they don't offer much help. Open for suggestions.
How about a part of a particular species of cockroaches, or perhaps cockroach or bat dung? Caves are covered in the stuff. If you watch the BBC documentary Planet Earth, there should be an episode on caves. The ground is literally alive with cockroaches, so much so that the baby bats living above have to be very careful not to fall because they will be stripped to the bone within the hour. I know it's kinda disgusting, but it's very much a cave environment and I honestly can't see websites selling cockroach dung... Or go more paranormal and go for something like cockroach wings (some do fly within a certain season I think, as I remember in HK we used to have flying cockroaches). There're other cool stuff living in caves. If you get into submerged caves, there're stuff like blind salamanders I think. If I remember correctly, there might be an angel fish... don't remember if it's called angel fish. They're blind fish that actually climb walls All this stuff I got off that documentary I told you about, it's worth watching. Basically, I'd go away from herbs and go ahead with cave-dwelling animals. Those are not things you'd find anywhere on the internet. PS. Snake venom, why don't you go for marine snakes? They're the most poisonous snakes on the planet I think, aren't they? They might breed in sea caves - don't remember if that's accurate, but worth checking.
There are definitely flying cockroaches. When I lived in Florida, we had ones as big as B52's. Make it up. All the shrooms you found on line are clearly known and catalogued. If one had some healing property it would probably also be known and patented and handled by GlaxoSmithKline. It doesn't have to be a real thing. Shroomus crypticus. Example: In Michael Crichton's Congo, the "special diamonds" they were hunting for in the jungle for their techno-badassery were found to come from a giant geode. Diamonds DO NOT form in geodes. DO. NOT. NEVER. EVER. The process that creates a geode is utterly different to that which creates the conditions for diamond formation. He made that shit up because the average Joe/Jane isn't a big dork like me who knows this stuff. Make something up!
Depends on the nature of the story. Is the guy a scientist/doctor type looking for something like Yew bark that Taxol is synthesized from but we've not yet made a synthetic version? Or has he been consulting with Shamans who tell him of a water source deep in a single cave that has magical healing powers scientists don't believe exists? It's in your story where you will find your answer.