Where do you find reference to Letters of Concern/Admonishment/Reprimand?

Such administrative actions are sometimes called "non-punitive measures." The use of non-punitive measures is encouraged and, to a degree, defined in the Manual for Court Martial, R.C.M. 306(c)(2), which states:

Administrative action. A commander may take or initiate administrative action, in addition to or instead of other action taken under this rule [e.g., NJP, court-martial], subject to regulations of the Secretary concerned. Administrative actions include corrective measures such as counseling, admonition, reprimand, exhortation, disapproval, criticism, censure, reproach, rebuke, extra military instruction, or the administrative withholding of privileges, or any combination of the above.

The only difference between an admonition and a reprimand is the degree of censor.
A reprimand is more severe than an admonition. As with counseling, admonitions and reprimands can be verbal or can be in writing. When they are in writing, they are often referred to as a " Letter of Reprimand," or a "Letter of Admonition," or sometimes a " Letter of Caution."

Unlike counseling, admonitions and reprimands are "censors." It means you did something wrong and you are going to be chewed out for it. Records of admonitions and reprimands can be filed and later used to justify more serious measures, such as nonjudicial punishment actions, administrative demotions, and administrative separations. So it's wise to set things straight and request later that a Letter in your file be removed because you have corrected the problem.

Are you asking for examples? or regulatory guidance?
If the latter, need to know the specific topic to be Concerned/Admonished/Reprimanded on...
is it DUI? Job performance? Hot UA? Excessive driving infractions? disobeying a lawful order and not taking prescribed medication? Violation of a restrictive profile limitation?

If you are looking for format, its AR 25-50. If you are looking for examples...I am not sure but maybe someplace like armywriter.com