You will have to contact the maker of the treadmill for a part like that, doubt anyone else will make it.
Would not think its that hard to make your own up, it just looks like 1/4" spade terminals and a standard pot, though what its value is ?
If you actually have the old one you should find the resistance value stamped or printed on its body.
Again a friendly call to the treadmill maker should give you its value, or it might be specified in the instruction manual, usually free downloads.