How can one obtain an adamantine weapon?

Question

In the Monster Manual, gargoyles, golems, helmed horrors, and xorns all enjoy either immunity or resistance to:

bludgeoning, piercing, and slashing damage from nonmagical weapons that aren't adamantine

Is it actually possible to obtain a nonmagical adamantine weapon?

• Although the Dungeon Master's Guide describes adamantine armor (p.150) and Daern's instant fortress (p.160) as being made of this metal, and provides an AC for destructible adamantine objects (p.246), there's no mention of adamantine weapons.
• The creation spell (PH p.229) can produce an adamantine weapon that lasts for a single minute.
• The true polymorph spell (PH p.283) can't turn a regular weapon into adamantine, but it might be able to turn a goblin into one for an hour.
• The only way to obtain a (somewhat) permanent adamantine weapon in the game seems to be to loot one from drow elves (MM p.127).
• A PC (perhaps even a drow) with proficiency in smith's tools could presumably craft a nonmagical adamantine sword during downtime at 5gp per day (PH p.187), except that unlike silvered weapons, there's no pricing for adamantine.

It's seeming like mugging an NPC drow is the only by-the-book way to tool up against gargoyles.

Answer 1

Find one as treasure / Go on a quest for it

By the book is a little odd to ask for here — the place to find an adamantine weapon is in the game world, not in some rule in the PHB that would give players unfettered access to powerful treasure.

For a veneer of by-the-book analysis, unenchanted adamantine items are listed as magic items in the DMG (pp. 146–8, 150). There's only armour listed, but adding an adamantine weapon to a treasure trove would be a trivial bit of homebrewing (that many DMs wouldn't even consider homebrewing).

So the answer is: go on a quest for adamantine, either in weapon form, or raw material form and then a second quest to find the legendary smith who can work “one of the hardest substances in existence.” Or mention the desire to your DM, and get lucky finding it in a treasure cache. That's how you get an adamantine weapon.

Answer 2

Xanathar's Guide to Everything addresses this by adding an option to have a weapon made of or coated with adamantine for an additional 500 GP above the weapon's base price. The relevant details, which are located on page 78, are quoted below.

Adamantine Weapons

Adamantine is an ultrahard metal found in meteorites and extraordinary mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons.

Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

Additionally, the Player's Handbook provides many classes with magical weapons or attacks by default.

• Pact of the Blade Warlocks can perform a ritual to make a mundane weapon their Pact Weapon which is nearly identical in function to that of the Eldritch Knight's bonded weapons.
• Circle of the Moon Druids get the Primal Strike feature which allows attacks made during Wild Shape to be considered magical for the purposes of bypassing immunity and resistance to nonmagical attacks and damage.
• Every full spellcasting class has access to at least one damage dealing cantrip, which bypasses immunity and resistance to nonmagical weapons.