It sounds like the OP might be confused. If you've "found" guano on the floor in caves, it doesn't randomly spawn there and bats don't randomly drop it. You have to feed the batterflies food so they can poop it out. What you've seen is probably someone fed the bat and never picked up the guano. So you don't have to hop around caves. Just find one that has bats and keep feeding the bat till you gather all the guano you need. There appears to be a pattern for how much guano the bats fly depending on what you feed it: 15 currants = 1 guano, 50 currants = 2 guano, 100 currants = 3 guano.
As JoJoJoey said they do randomly drop guano (and often in set spots).
The best way I've found to get it is feed them 3 planks and you'll get 1 guano, I think something similar works with eggs? You can find different food-guano combos on the wiki :)
There is a certain spice that will give one drop for one spice, every time. You feed it three of them and you get two, so 1:1 is the cheapest way to go, but also the most time consuming.
I often whip up a few batches of pickles. Pickles are pretty easy to make (1 cucumber, 1 salt, 1 olive oil), and stack up to 40, which is conveniently the same stack size as the guano itself. Each pickle is worth 21 energy, so they're worth one guano each.
I just position myself beneath a bat and use the mouse. I click on the mat, click feed, click on the pickle, and then "just one". Over and over and over. Try not to think about how a bat is pooping on you the whole time. Also, just be careful to pick up your guano from time to time, as passing people will often steal some. (Either they don't know what you're doing and think it's free guano, or they just don't care because hey, free guano.) If you're good, it takes ~3 minutes to go through a stack of 40 pickles/guano this way. I often do about 80-120 per trip every couple weeks, and that keeps my garden going strong.
@connfowsled: Friend has wood trees that we harvest from, but I don't actually have any skills that use planks. Hence they make good bat food for me.
I like the sound of the pickle but it does require olive oil which you need to buy. I'm curious about this spice that nanookie mentioned, which one specifically?
There are two things I feed to Batterflies:
3 stinky cheese= 1 guano
I make the cheese in large batches when I am on a no-no rush at home. As I have no other use for butterfly milk, this costs me almost nothing.
1 rich tangine = 3 guanos
It is a bit more of an investment but with much greater returns.
It shows that the food's street value determines the amount of guano drops. Therefore, rich tangine is the the most efficient poopmaker because it has value just a little over 100 currants.