You would need to buy the software from a collector or something, Xerox definitely doesn't have the software available.
They do occasionally come up for sale, but due to them being older (1985) than the Macintosh, it isn't very often.
As for manuals, somebody would need to scan one, Xerox never did since the tech didn't exist at the time. I checked some internal archives but they only go back to the early-mid 90's and bad scans of advertisements in the 70s-80s.
I know this may be a stab in the dark, Was any of the documentation archived such at the 645S Memorywriter training manual? Lastly is there a source for 645S software disks?
Thanks
Don Resor