Though you may see their’s written even by native speakers, it is inc…

[davidmiller]

…orrect. Theirs should never have an apostrophe.

Theirs is the third person plural possessive pronoun – it replaces their + noun. Is this yours or theirs? He found a book – is it theirs? I can’t find my keys, but theirs are on the table. Theirs is a better idea. Theirs is over here.

Ref: http://www.elearnenglishlanguage.com/blog/english-mistakes/theirs/