I believe that sight is a restricted item, meaning a new one has never been surplussed
I think you're thinking of the M-53a1 sight. They look pretty similar, and a fair number of M-53a1's are around. They turn up on ebay/gunbroker often enough.Those sights have been out for quite some time and have been surplussed over the years. They were intended for the 81mm M29A1 I believe but will fit the M2, M19, M224 60mm guns as well as the earlier 81mm mortars. They have finer graduations and control on movement than the old M4 sight as well as a much better sighting system. Its much simpler to line up with the graduations on aiming posts.
I used on on my M2 60mm mortar once to see how well it worked and its nice.