I doubt I’m the first person to ever come up with this mechanic, but I can’t remember ever seeing it anywhere else. Anyway, I’ll just throw it out there…
This is a simple tool I use sometimes when a PC asks if something is available or nearby; something that very well could be, and could be useful if it were, but not something I had given any thought to. A good example would be asking if there is a payphone, data terminal, or a what-have-you nearby in a modern or cyberpunk style setting, or maybe a particular flower or herb in a wilderness setting. Of course you could simply say yes or no and move on, but sometimes the presence of such a thing could make a real impact in the way events play out and simply saying yes or no on a whim often doesn’t seem “fair.” This is where the Percentile Percent Chance comes in. I simply roll percentile dice, out in the open, and whatever number is rolled becomes the percent chance that the asked about item is in the immediate vicinity. The player then rolls percentile dice, attempting to roll equal to or less than whatever the percent chance is. And that’s it! I roll two dice, they roll two dice, and we’re back in the action.
Of course, you can get more involved with it if you want, maybe adding or subtracting a flat percentage from whatever the die roll is to represent the presence of the object being more or less likely (that plant does grow in this region, but it’s rare, etc.). I would make this sort of adjustment sparingly though, otherwise you might as well just say yes or no.
You may also be thinking “Why not just make an herbalism or an urban knowledge check?” I would say that if the situation is such that the character has the time to search for something then sure, go ahead and use a relevant skill, or just say they find it. I use this mechanic when the need is immediate (I need to jack in now! Is there a terminal nearby?) and the presence or absence of the object/thing in question is both plausible and could affect the story in an interesting way.