Another’s is the correct possessive singular form of the word another. Anothers is incorrect because there is no plural form of another. Anothers’ is also not possible because there is no plural possessive of another. Another means one more and therefore there is no plural.

The possessive form of “another” is another’s. Why not say anothers’ or anothers? Let’s find out! Related: Others, Other’s or Others’: Which is Correct? The term “another” is a pronoun or determiner that usually refers to an additional person, thing, or idea.

In fact, in both cases, the correct word is another, so you should say "I looked for another customer service rep" and "Do you have another one I can look at?" Why? The word another is used with singular nouns.