As mentioned in this question, I know that "many a year" is literary and equivalent to "many years".
But I stumbled upon this phrase:
'For many's a year now,' he said, 'we've been running a source inside the police. [...]'
I suppose it has the the same meaning as above. But why is structured like this? What does that 's mean here?
This is an example of English vernacular. It sounds like a "rural" dialect of some kind, although without more dialogue I can't specify from where, exactly. As you guess, it means the same as "for many years now".
Similar example:
"Many's the time I done tried to get him to open up and talk about it, but he never did."
Because it's a dialect, I wouldn't recommend using it unless you were trying to imitate someone who talked that way naturally.
