I'd like to ask the following question about what the "standard"/"expected" way to do something in D&D 5e is, but I feel that it'd likely be deemed opinion-based or unclear with regard to the meaning of "expected." Is the question acceptable for the main site, and, if not, how can I modify it so that it is?
What is the expected way to acquire costly material components?
If a spell's material components include any items with an indicated cost (such as the pearl worth at least 100 gp needed for identify), those items cannot be satisfied via a component pouch or spellcasting focus but must specifically exist in the caster's inventory. This raises the question: Where are characters expected to find such items? The DMG does not seem to address this (and if it does, that would likely be an answer).
There are two basic options: either cities have stores where you can buy these components at exactly the right prices, or else players can find costly components in treasure piles. The first option is boring, and the second option isn't supported by the treasure rules in the DMG. Said treasure rules only acknowledge treasure that is made up of coins, gemstones, art objects, and magic items (DMG p.133). Some spells do need gemstones, though, and while identify's pearl can be found in a treasure pile, that doesn't work for all such spells:
Some gemstones are listed in the tables at insufficient price; e.g., awaken requires an agate worth at least 1,000 gp, but all three types of agates in the treasure table are only valued at 10 gp each.
Some spells require gem dust, but there's no mortar & pestle listed in the adventuring gear that could be used to grind down a gem to dust — and even then, there are price issues; continual flame requires ruby dust worth 50 gp, while a ruby is worth 5000 gp; does grinding down a single ruby give you enough dust for 100 castings?
Some spells require gems that aren't even in the treasure tables, like create undead needing a black onyx stone worth 150 gp. (There's an "onyx" listed, though, and while its description states it can be black, it's still only worth 50 gp).
Plus, there are still all the other spells whose costly components don't resemble any DMG-listed treasure, such as the incense and sacrificial offering together worth 25 gp needed for divination.
While flipping through the DMG to write this question, I encountered a third option: crafting. Page 46 says, regarding the forked, metal rod worth at least 250 gp required for plane shift: "Crafting the fork is expensive (at least 250 gp), but even the act of researching the correct specifications can lead to adventure." Putting aside the need for research that is specific to this spell, this answers some questions but raises more: Can all costly components be crafted? How long does the crafting take? Are skill checks, tools, or proficiencies needed? Does the character need a formula for crafting as is needed for magic items? Is the money spent on anything specific from a special store, or does the player just subtract it from the number of gp on their character sheet even if they're currently in a dungeon nowhere near civilization?
Or do we just go with the really boring fourth option: The first time you cast a given spell with a costly component (and every time after that if the component is consumed), do you just subtract the cost from your gp and call it a day?