I contacted Lipsey's and they informed me the rear sight is made in-house by S&W, and that all their 642UC rear sight production is allocated to 642UC production.
Perhaps if 642UC sales slow down the sight alone may be offered?
642UC Rear Sight
I contacted S&W asking to purchase a 642UC rear sight. Their reply received today -
"Thank you for contacting Smith&Wesson customer service. We unfortunately do not have the rear sight available for purchase"