You can try MSC, McMaster-Carr, Grainger, mebbe goggle "power transmission" supply houses. Here in Brooklyn, there is Northside Power Transmission, but a long shot for this particular part.
Easy enough to make if you have a lathe, or even better, a friend with a lathe. :) Take 1.25" solid brass or alum or steel, tap one side, drill out the other, tap yer set screws, and voila, adapter.
They might also make something in a LoveJoy type coupling, you know, with the rubber interlocking fingers. One half would fit the threaded, the other half the unthreaded. Proly sold in separate halves, precisely for mixing/matching.
Curious as to why you are gear reducing your grinder. Seems to have possibilities! :)