A small experiment to get random textures in the same material.
maybe it can help someone.
I don’t know if this is the best way, however this works.
The material load one image with a grid of 3x3 texture and only uses one, random.
Source images example:
And a view of the node editor:
There is a colorRamp node to change background color for each textures (always the same color for the same texture); for my render test I used a greyscale but in the blend file there is also a “colored” version.
here the blend
The same method can be used with denser grids (4x4, 5x5…), but I am too lazy to try 
ciao!
