You're not just going to buy that, highly unlikely anyway. Confirm its 11/16-8 LH acme thread.
You can buy that tap yourself and some 932 bronze to make it.
Or contact the guys making regular 16 crossfeed nuts. They atleast have the tap, or a setup to cut that thread. Send them your nut, pics, dimensions whatever. They'll work it out. But cost as a one off will be higher.
The screw probably is worn too, but a nut will help.