I'm looking for room tone recordings of a large library/old building with a marble/granite staircase, can anyone point me into the direction of any sample libraries?
You may have some luck with the Rabbit Ears Audio REA0013 set. There's a few good ones in there which you can layer, although I feel some are riddled with preamp hiss. I believe Soundeffects.ch has a roomtone set, although I'm not familiar with it.
Likely to achieve the sound you want, it will involve layering different roomtone textures and EQing them to shape the space you seek. Generally the space you speak of will have a wide image, a lot of lo/mid-tow air as a "weight", without much of the hissy/high end at all. Usually it's the small resi rooms which have not much mid and low end and are mistly the "clean air" hi/mid-high hiss/presence.
So I think it's going to be a combination of checking out some sets, like those I mentioned, and creatively shaping different timbres to achieve the exact quality you seek.
Tim Prebble once undertook a massive "globalsourced" library dealing exclusively with room tones but don't believe it ever came to fruition. Perhaps he would be a resource?
Otherwise I concur with @Stavrosound; you'll more than likely need to create your own to match what you're seeing on the screen.