###There is nothing special about material components as loot or purchasable items. They are loot, and/or purchasable items.

I'm definitely open to any official publication or comment, but my impression of the official material I'm aware of is that the DMG already contains all the information needed to offer price-bearing material components as loot or goods in a shop, and it would be redundant to list them in the loot tables explicitly.

My loot-related arguments follow, with other options at the bottom:

Position 1: All Loot is Currency

The individual treasure and treasure hoard tables in the DMG classify all treasure by its cash value. The rolls you make using these tables determine a bounded range of how valuable the loot is in terms of cash (for coins and non-magical items) or rarity which directly correlates to a bounded range of cash values (for magical items).

Some items are intrinsically valuable to an adventuring party of D&D players, like weapons or armor, and the cost of those items is detailed in the PHB. For the rest, they (typically) have no mechanical properties, uses, or features outside of the amount of money players can get for them. What are players going to do with a carved ivory statuette or bronze crown (250 gp art objects from the DMG treasure tables), besides sell them? What are they going to do with a gemstone that isn't used as a material component for something?

The treasure tables aren't there so much to tell you exactly what to give out to your players but instead to suggest an appropriate cash value of reward relative to the challenges your players tackle. If an individual monster wouldn't carry coins, the DMG explicitly says to substitute non-coin treasure of equivalent value. Generic treasure isn't meant to be specific things, it's meant to be worth a specific amount.

Therefore, all generic loot is fundamentally defined by its cash value and not by any specific properties that loot may have.

Position 2: Non-cash Loot is Not Inherently Valuable without the Treasure Tables

It's really hard to assess the cash value of a random object, and so for the purposes of awarding loot (other than coins) the DMG treasure tables assign a cash value to organize it in keeping with (1), above.

Is the DMG really suggesting that any Alexandrite gem that exists in the game world is worth exactly 500 gp, with no variation in size, quality, or clarity? If a monster found one that was worth 450 gp, would they immediately destroy it or add 50 gp of value to it? Perhaps-- I can't guarantee that it isn't that way. Is it saying that all small, gold bracelets are worth exactly 25 gp? Again, perhaps.

But I submit that, as these generic loot objects have no properties (with respect to the mechanics of the game) besides a cash value, the actual items in the tables exist only to be worth set amounts of currency, and what players might get is based only on how much the items are worth.

Position 3: Price-bearing Material Components Are Already Worth Fixed Amounts

Here's where the above two arguments meet the question. Price-bearing material components don't need to be slotted into the random treasure tables because they already have values assigned. A "diamond worth 1000 gp" is already 100% specified in terms of what might appear on a loot table. As far as I'm aware, nothing in the loot tables is listed with a price anywhere else in any published material.

The DMG doesn't need to list a diamond worth 1000 gp on the loot tables because you already know how to place it: treasure worth 1000 gp. It's the inverse reason that an Alexandrite gem needs to be explicitly assigned a place in the treasure tables (it has no discernable value otherwise).

There is simply nothing special about the identity or composition of specific items of loot, since their only meaningful property is cash value.

Position 4: The Treasure Tables are Neither Exhaustive nor Restrictive, and Items Can Belong to Multiple Descriptive Categories

There is nothing in the DMG saying that treasure must be randomly generated, nor that awardable loot must be on those lists, nor that items can't belong to multiple categories. An art object of at least a material component's cost might contain that very material component, such as a fork for casting Plane Shift being part of a larger sculpture.

But fundamentally there is nothing suggesting the tables are all of the loot a DM can award, and so the idea that they restrict a DM from awarding other things is unfounded. Particularly given (1) and (2), above, there is no reason to think that any properties of the loot matter except its cash value. And so a DM already has all of the information necessary to assign price-bearing material components as loot while being exactly consistent with the guidelines from the random loot tables (should that be of interest to the DM).

###Non-loot Considerations:

I'll take each of the four explicit ideas in the question in turn:

1. Buying them in a shop:

It's already expected that magic-related goods are available in stores, though the rarity of those items varies by city size and magic-related commerce. This is not so different from weapons and armor. That you find this approach boring (understandably!) doesn't remove this as an option.

2. Finding them in treasure piles:

Addressed above.

3. Crafting them:

I don't see any reason this wouldn't work with the existing rules, provided that the character is capable of producing it and the cost + added value of PC labor is equal to or greater than the listed price of the component.

This is a bit fuzzy for some components, though. If a PC finds a diamond worth 750 gp, can they cut, polish, or otherwise modify it to be worth 250 gp more? Per published information on a 1000 gp diamond as a material component, maybe. All that matters is value, and that value itself seems to be arbitrary.

4. Subtracting the GP cost at time of casting:

This one is an explicit no. Material components can't be swapped around (except for those spells which offer a selection of components to use), and so if you need a diamond worth 1000 gp you need the diamond, not just enough cash to buy one.

Material components without a listed price are assumed to be always available to a character (via a component pouch) or unnecessary (with an arcane focus). Price-bearing material components are explicitly different in this regard, and so no PC should be able to just assume that they have one as long as they have enough money to buy one.