How do I find this detailed 'noise' texture?

A Hard Mix Pattern Layer might serve as the basis for something … 

I am not sure which texture you mean – the one in the background behind the persons or the overall, fine texture that might have resulted from indexing? 

I've spent a good ten minutees trying to get a close match and not done well.  I can't help but think that Filter > Stylise > Emboss would come in to it as near to final step, but you need to start with the right noise texture.  Camera RAW used to have my favourite noise generator with it's grain texture, but they have changed it.  It is still there under effects, but we can't control the grain size now.

Whatever you try you should probably start with adding noise because other effects need something to work with.

Whoops, you'll need to fill with something like 50% grey to add the noise to.

Then try and work with one of the textures and then use emboss.  That was always leaving the effect darker than your example, so you'd need to use curves or levels to fix that.

If you find you are adding colour at one of the stages, kill it stright away with saturation.  If you make your layer a Smart Object from the outset, you can go back and fine tunes.

I also tried using a rough concrete texture via Google, but it was not much better and it felt like cheating.

Something else you could try is starting with a dramatically oversized canvas and using Render Clouds.  After the first render use Ctrl Alt F which gives you a more dramatic effect.  Then downsize with Free Transform to make the pattern nearner to the clump size you need, and then try Emboss.

Good luck and show us what you come up with