Try Turnbull,,we had a large supply of Ithaca SxS parts when I worked there, but that was many yrs ago. Who knows what the story is now.
Most were NID, but there were others too.
All came right from Ithaca Gun Co when the Plant at Ithaca NY closed up.
They may just say no because they don't want to sell any parts separately. That's common in the Gun FixIt and Restoration trade.
You can add matrl to the old part by welding or even brazing or silver soldering (hard solder).
Use a lap joint if brazing or hard soldering for extra strength.Then file to shape & fit. You have the rt side part to show you the shape.
It's not to hard to make one from scratch if you want to.