I think the only way to get one would have a gunsmith that is proficient at welding and such make one. I've never seen a large loop on an 1873. I've seen plenty on 1892 clones, Marlin 336 actions, and on even 1894 Winchesters though.
Try Brownells for the loop. I think that a '94 large lever loop cut off and welded to a '73 lever base would probably work well. I installed a large loop on my '94, looks great, fun to shoot, and as stated above- slow as molasses.
Numrich carries Winchester levers, but they are out of stock now. Brownells only carries Marlin levers. Watch the gun auction sites, they are only bringing $50.00 or so used.