Edit: it seems i got confused and thought the person who asked about herbs was asking this to ultimately make a potion. If u only want an answer to how d&d handles finding herbs scrol to the bottom of this comment otherwise if u also want to know how to make potions start from above.
I kinda found out why so much people got trouble coming up with the right way to craft a healing potion using the herbalism kit. The above is very detailed but flawed at one point i will place a link where a kind sir explains the exact cost and days spend on making a healing potion and why its different than the above. (https://www.reddit.com/r/DnD/comments/5lon4k/herbs_and_where_to_find_either_a_list_or/)
The reddit post also points out the deal with ingredients... Hope this helps (because really no wonder people think making healing pots isn't worth it because of 10 days downtime AS THIS IS WRONG)
Duo to the request of placing the answer here instead of using a link to an external source I will just place the formula for making magic potions/consumable items like scrolls and the much asked healing potion below.
So first I will state 3 things which you can look up in the DMG and the PHB so my sources are none other than these 2 legal pieces of work.
healing potions are magic items even if the PHB sorts them in adventure gear.
non magical item crafting and magic item crafting are 2 different things. The first can be found in the PHB while the second can only be found in the DMG.
MOST IMPORTANT in the DMG page 135 there is a part that says that consumable magic items--I repeat: consumable magic items--are half of the price of a comparably-rare permanent item. Thus the crafting cost of a potion of healing (consumable, common) is 50gp, compared to the 100gp crafting cost of a permanent common magical item.
So now finally for the FINALE: the much asked-for formula for the healing potion crafting time and details.
Magic crafting rules give you 25gp crafting value each day instead of 5gp for normal crafting rules meaning you can finish a healing potion in 2 DAYS.
This makes a lot of a difference compared to what most people think 10 days by normal crafting rules. So to sum it up crafting a potion if healing will cost you 2 days of downtime and 25 gold pieces.
One thing i was confused about is that herbsHerbs are not really a thing in D&D5eD&D 5e: there is no list of existing herbs nor are there specific recipes for potions. So
So what does the PHB description of the herbalism kit mean with herbs? Its just a basic description the crafting rules want you to literally cough up the ingredients in hard cold cash. You don't need to forage for herbs or buy them from a store you just need to pay for it.
Even though there are spells like hallow which needs components like "herbs" and some other stuff with a worth of 1000gp meaning an arcane focus or a comp pouch wont work.
You would think that there would be a section about finding herbs or buying them or anything specific -- sorry to say there is nothing of the sort. So
So either the DM needs to make something up like you going to a store saying "Hello, I want to buy 1000gp worth of herbs," and he gives you a bag of herbs.
Or he can make up some homebrew recipe list which(which would not reallyreally be homebrew, because Wizards ofthese things are supposed to be made up by the Coast left this open on purpose?DM).