The Code

In order to have Lawful characters, you first need to have a law. Most works provide that law in the form of The Code.

The Code is basically any code of honor or conduct that governs a certain group of people. Codes can be written down or oral tradition, they can be short or long, or practical or philosophical; the gist of the trope is that it has the more lawful characters of the cast mulling over which course of action would be "true to The Code".

Orders and Ancient Traditions are very likely to possess one of these. The Obstructive Code of Conduct is the most important (or just first) article of The Code. The Commandments are a very brief Code presented in the form of an enumerated list. The Big Book of War is a written-down Code that pertains specifically to war.

Film

 * The Jedi Code in Star Wars.
 * The "Code of the West" is sometimes mentioned in The Western genre.
 * The Pirate Code of Pirates of the Caribbean is more like a set of guidelines rather than actual rules, but it still works.
 * The Arthurian code of chivalry, as referenced in Dragonheart

Literature

 * The Code that the barbarian heroes (and their opposite numbers, Dark Lords) live by in Discworld.
 * The Stormlight Archive has the Alethi Codes of War, and the Immortal Words of the Ideals of the Knights Radiant (the first Ideal being "Life before Death, Strength before Weakness, Journey before Destination." the others being different for each of the ten Orders).
 * Geralt has one of these and will often bring it up when he refuses a job or stays neutral in any given conflict.

Live Action TV

 * The Bro Code from How I Met Your Mother.

Tabletop Games

 * GURPS has various Codes of Honour as disadvantages. While many of them are connected to specific organisations, there's also things like the "Pirates' Code of Honour" and "Gentlemen's Code of Honour"
 * The Oath and the Measure of the Solamnic Knights in the Dungeons and Dragons Dragonlance setting.
 * Forgotten Realms has too many specific organizations. Even paladin orders of the same god don't have exactly the same charter: they were founded by different leaders for different goals. Of those published, there's The Code of the Harpers (they got a separate Sourcebook).
 * The Paladin's Code from Dungeons And Dragons is intended to be a non-denominational version, and the paladin must adhere to it or lose his abilities. Unfortunately, sticking to the letter of the code will result in a Lawful Stupid character. They're not even allowed to render aid to chaotic characters.

Toys

 * Bionicle has the Toa Code, which is basically Thou Shall Not Kill.

Video Games

 * The code of the Asari Justicars in Mass Effect 2.

Western Animation

 * Da Rules in Fairly Oddparents.
 * The Code of Thundera in Thundercats: "Justice, Truth, Honor, and Loyalty."

Real Life

 * The bushido code of the samurai class.
 * The chivalric code of medieval knights.
 * The Mafia code of Omertà.
 * The Hippocratic Oath for doctors. Making this trope Older Than Feudalism.