Getting ready to battle a horde of dragons. What are the best magic weapons to fight dragons? Found arrows and some charms against dragon breath. What non-homebrew items could help?
Generally, a Dragon Slayer sword and Dragon Scale Mail.
A Dragon Slayer gives a significant damage boost (3d6) against dragons, while Dragon Scale Mail gives advantage on saves vs breath weapons and Dragon Fear. If it's the right color, Dragon Scale Mail can also give resistance against the dragon's breath.
If you know the dragons' color[s], there are some other items that can give a large advantage.
If you're fighting a red, gold, or brass dragon, for example, Efreeti Chain will grant you immunity to its breath weapon and any other fire abilities it has.
If it's a green dragon, a Periapt of Proof against Poison will make you immune to its poison breath and bite.
The Dragon Slayer sword
A rare magic sword ("any sword") from the DMG, p. 116:
You gain a +1 bonus to attack and damage rolls made with this magic weapon. When you hit a dragon with this weapon, the dragon takes an extra 3d6 damage of the weapon's type. For the purpose of this weapon, "dragon" refers to any creature with the dragon type, including dragon turtles and wyverns.