Noble recruits are the second unit tree available for each faction. Rather than the main unit tree which is branching, noble unit trees are linear and end with each faction's elite unit. The difference between noble and normal troops is quite drastic, especially when maxed out.

For example, the Imperial Elite Cataphract is lvl 31 with 220 One-Handed, 280 Polearms, and 220 Riding. The 5th tier normal imperial units (legionaries, palatine guards, etc) are all level 26 with top skills capped at 130. The 5 levels and 100 skill points make a large difference, so having large amounts of noble units is desirable.

Noble recruits can be found in villages with powerful landowners. Any village with notable characters of type landowner and power greater than 200 has a chance to spawn noble units. The higher the power of the notable character, the higher change they have noble units. I have found that anyone with 300 or more power tends to have several noble units and some that are already upgraded several tiers.

Another way is to defeat an enemy lord, and hold his noble units prisoner until they are willing to convert and join your party.

Additionally you can obtain noble units by converting bandits into noble units using the Disciplinarian skill unlocked at leadership 125, which I have now confirmed works.

Hillmen -> Brigands -> Highwaymen -> Vlandian Champion
Freebooter -> Forest Bandit -> Battanian Fian
Bedouin Rover -> Nomad Bandit -> Harami -> Aserai Faris
Steppe Bandit -> Marauder -> Raider -> Khergit Kheshig
Sea Raider - > Sea Raider Warrior -> Sea Raider Chief -> Sturgian Druzhinnik

From what I can tell there is no way to get Imperial noble units from bandits.

At the moment, the only way I know to increase the power of landowners is through quests. Since quest spawning is fairly unreliable and doesn't increase power significantly (about 5 from what I can tell) it is difficult to actually have more noble recruits spawn in your map.

I still recommend doing quests for powerful landowners though as the extra power given to them certainly doesn't hurt and you also unlock more recruiting slots.

There is also the "inspiring warrior" skill available at leadership level 200 which has 20% faster noble recruit replenishment. I have not been able to test this skill and see if it works. To farm your leadership skill create parties with your companions (found in clan tab) and summon them to fight for you. You have to be part of a kingdom to do this, but it costs zero influence and allows to lead armies constantly which increases your leadership skill.

I have written down the villages in my world that meet the following conditions:
A) Have 2 or more landowners with 200 power
OR
B) Have at least 1 landowner with 300 power

These villages are the best for finding noble units, and I recommend visiting them frequently. If you would like to increase your odds of finding noble units, recruit all the troops from the powerful landowners (even if they are regular recruits) to free up slots for noble units to potentially spawn.

If you have additional information that might help this guide, feel free to leave it in the comments.