They can be found in a random encounter.
Noble on the HorsebackEdit
Sometimes, the Dragonborn can find a noble traveling to Solitude on a horseback with his bodyguard, an Imperial Legion member. He will be rude and threatening to tell his guard to attack. However, he will be less hostile if his guard is dead. It is also possible to try to rob him, however he will only run away and his bodyguard will turn hostile.
Another random encounter includes two nobles of both genders, traveling along with their guard. Their destination is Solitude. They hope to make it for the Vittoria Vici's wedding. It is possible for the Dragonborn to approach the male noble and try to rob him, however he will run away and his bodyguard will turn hostile.
Additionally if the quest "Bound Until Death" has been completed, the Dragonborn can find the very same pair of nobles with the female one walking barefoot, complaining that they didn't make it to the wedding and that she destroyed her boots.
It is also possible to meet two nobles escorted by two Imperial guards. They carry some valuable items and it is possible to try to rob them in order to get them, however it will only make them flee and their guards fight.