I can't find the size but the P/N is 9991164180 & lists for $3.03 in the parts list. Call Merlin to see if he has any in stock. He may also be able to tell you what size it is.
BTW, this info is on pdf page 8 of the parts list (shows page 1760 on the page). It's item # 36 on image 4628195 of that page.
Hopefully you can get the castle nut withoug buying the whole ball joint.
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