That's not really something you should be seeking samples of. It has to be different than other people's letters by necessity (since you are different, and the context in which the letter writer knows you is different), and ethically since folks are really not supposed to see their letters (since they are waiving rights to see them) it is foolish for anyone to come out and say "here is a letter I wrote for myself". It puts both that person and the signer at risk.
But in general: it should say who the person is, how the person knows you, that they have had opportunity to observe you in settings that might reflect on how you will conduct yourself in a professional setting, and that they find you to be among the top such students they have worked with and that they would recommend you strongly.