When the Aldaires took over the cotninent more than 100 years ago, noble families who resisted were given the option to submit to Aldaire rule. Those who did not show fealty to Aldaire rule were stripped of all lands, posessions, and titles.
The following is a list of former noble houses who are still present in some form or other.
Maltarr - Formerly a major house in the North
Falin - Formerly the ruling house in the East
Altross - Formerly a major house in the Southeast
Eurgos - Formerly a major house in the East
Sanum - Formerly a major house in the South
While there are other major and minor families of former nobility, few of them are still in any way in tact. The above list consists of families who have managed to survive despite being stripped of lands and titles; they've turned to merchants, importers and exporters, and organized crime.
Unfortunately you cannot be a knight of one of these families (unless posing as someone else, I suppose)