I need to find an exterior door threshold for an over hundred year old house. The jamb is between 9 to 10 in. wide.

For something like that, you would be well off to find a cabinet shop that could make you one because a jamb that wide will be very hard to find.

Choose White Oak for species to get another possible 100 years.

You could also buy an oak stairtread, cut it to size and angle/bevel the front and back. If you have the tools, probably less than $20.