Midway usually has them

by FOG, Tuesday, September 03, 2013, 01:03 (4104 days ago) @ Slow Hand

They're out-of-stock at the moment, but say they'll have more in early October and backordering is 'OK' (which usually means the item is actually somewhere in the pipeline).

Smith & Wesson Rear Sight Assembly .086" Black J-Frame @ MidwayUSA

HTH

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