The word “another’s” is correct when you want to say something belongs to another person. For example, “This is another’s book.” On the other hand, “anothers” and “anothers'” are not correct in any context.

The possessive form of “another” is another’s, and there is no plural form. When referring to indefinite pronouns such as somebody, anyone, or nobody, you would use apostrophe +s in the possessive form (e.g. someone’s, anyone’s, everybody’s), even if you are referring to multiple people.