There are two official optional rulesets for crafting
Both are optional and require that you work with your DM as to if they're an option and how they'd work. In either place that these rules appear, they are optional variants to the base rules.
Dungeon Master's Guide
The first official optional rules released appear in the Dungeon Master's Guide in chapter 6 as part of the "Downtime Activities" section:
Crafting a Magic Item
Magic items are the DM’s purview, so you decide how they fall into the party’s possession. As an option, you can allow player characters to craft magic items.
Within these rules, there are the following requirements:
- a formula that describes the construction of the item
- the character must also be a spellcaster with spell slots and must be able to cast any spells that the item can produce (and expends the relevant spell cost each day during crafting)
- the character must meet a level minimum determined by the item’s rarity
- a required gold and time cost to craft the item during downtime (consumable cost half as much as other magic items)
\begin{array}{c|c|c}
\text{Rarity}&\text{Cost}&\text{Minimum Level}\\
\hline \text{Common}&\text{100 gp}&\text{3}\\
\text{Uncommon}&\text{500 gp}&\text{3}\\
\text{Rare}&\text{5,000 gp}&\text{6}\\
\text{Very Rare}&\text{50,000 gp}&\text{11}\\
\text{Legendary}&\text{500,000 gp}&\text{17}\\
\end{array}
Xanathar's Guide to Everything
An alternative ruleset is part of the optional variant to downtime as a whole (and one I personally think is more fleshed out) that appears in the supplement Xanathar's Guide to Everything in the "Downtime Revisited" section of chapter 2:
Crafting Magic Items. Creating a magic item requires more than just time, effort, and materials. It is a long-term process that involves one or more adventures to track down rare materials and the lore needed to create the item.
Within these rules, there are the following requirements:
- a formula for a magic item in order to create it
- an exotic material to complete it. This material can range from the skin of a yeti to a vial of water taken from a whirlpool on the Elemental Plane of Water. Finding that material should take place as part of an adventure. (the suggested challenge rating of the creature to face, though not necessarily harvest the material from, appears in the table below)
- whatever tool proficiency is appropriate, as for crafting a nonmagical object, or proficiency in the Arcana skill.
- a required gold and time cost to craft the item during downtime (consumable cost half as much as other magic items). Do note that downtime rules as a whole are revised when using the Xanathar's Guide to Everything rules
\begin{array}{c|c|c|c}
\text{Rarity}&\text{Challenge Rating}&\text{Time}&\text{Cost}\\
\hline \text{Common}&\text{1-3}&\text{1 workweek}&\text{50 gp}\\
\text{Uncommon}&\text{4-8}&\text{2 workweeks}&\text{200 gp}\\
\text{Rare}&\text{9-12}&\text{10 workweeks}&\text{2,000 gp}\\
\text{Very Rare}&\text{13-18}&\text{25 workweeks}&\text{20,000 gp}\\
\text{Legendary}&\text{19+}&\text{50 workweeks}&\text{100,000 gp}\\
\end{array}
Wayfinder's Guide to Eberron p. 46 has a section on "Creating Magic Items" with rules adapted from Xanathar's Guide to Everything and prices aligned with its "Buying a Magic Item" downtime activity. It suggests that House Cannith would be able to make many items at a much lower price, but common items look like they would be fairly priced at around 50 gp (probably more).
Xanathar's Guide to Everything p. 126 has the "Buying a Magic Item" downtime activity which is as far as I know the only way to buy magic items by RAW. The asking price for a Common magic item is listed as (1d6+1) x 10 gp, or half of that for a single-use item like a potion or a scroll. This isn't counting the 100 gp and one workweek of downtime spent finding a seller.
Normally there wouldn't be any guarantee that the particular item you're looking for will be for sale. To find a particular Common magic item your Charisma (Persuasion) check needs to beat a DC 10, with a +1 bonus for each additional workweek or 100 gp spent searching (up to a maximum bonus of +10). However, because Eberron is a high magic campaign XGtE suggests adding a +10 bonus to the check result which would mean that you can automatically find any given common magic item for sale.
Best Answer
1. None of the named magic items are found in the index...
... annoying, but it seems only the categories of magic item (rings, rods, staffs, wondrous items etc.) are in the index, not the specifics. Instead of using the core books to search for specific items, I recommend using one of the SRD websites - I use 5thsrd.org to quickly search for individual rules.
2. By default, you can't buy most magic items
Default 5e assumes that "magic shops" pretty much won't exist, and even goes on to suggest that potions and the like be bought at alchemist's shops instead.
From the DMG, page 135:
and further:
The Bag of Holding is an uncommon item, therefore in the 101 - 500GP range. At this point, it is at your discretion.
Magic items in 5e
Unlike previous editions (particularly the 3/3.5/4 era) 5e is designed to depend much less on magic items. Together, the:
...ensure that high level 5e characters are powerful because of their raw skill and ability, not the plethora of magic items they happen to be decked out in. This was a specific design decision going back to the very first D&D Next playtests. (As a side note, having your party find that Bag of Holding in some dusty and forgotten attic is also more rewarding than just stopping by the local Bags of Holding R Us and grabbing a few!)