Measure the thread diameter on the bit left.
Not sure if you mean the lock nut to the rod which the track rod end screws onto....which will likely be about 12mm??
Or the thread on the end itself, above the taper...10mm??
Whatever the diameter is, reasonable guess that this will be a standard metric thread so if 12mm, call it an M12 and that is the size you need..... you can then search online or better at a local hardware store and find a nut.
If for the track rod end itself, make it a Nyloc (self locking nut)
For local places, search online for nuts and bolts in your area...or use fasteners as a search term.