Just measure the dia of the stud and count the threads over 10mm distance. You should be able to match it up to a nut on this website. Try to get the highest grade nut they have, 10.9 if possible. Should be under a buck.
Probably a M10 or a M12. Be aware that they do come in multiple pitches so get the right one or one of each.
Hard to Find Metric Fasteners and Metric Screws with No Minimums