Where can I get a sprocket that works with a standard bicycle chain that mounts onto a 5/8" bore rod?
If your grain mill is a prototype, I would make a sandwich of steel plates (ie really big washers) and clamp it all together.

        | &lt;-- sprocket
      | | |  &lt;-- two steel plates
   ==||=|=|=|======= &lt;-- shaft
      | | |
        |


Getting the alignment perfect might be hard and require tweaking, but this temp solution allows you to undo it and make adjustments.

Consider welding it in place. - this would have to be perfect, and it does make replacement of the cog difficult.  But it won't move or slip on you!



Final thought - if you don't want a rigid shaft, consider cannibalising a freewheel or freehub.  I'd consider slapping a whole freewheel cassette on, and using an appropriate gear.  This way the machinery can move forward on the freewheel but not backward.  Clamp it up between two cone nuts on the threadded shaft.