Very rarely is there only one of anything made for movies, they need stunt versions, hero versions, etc. Add to that screen time, photos, and any notes you can find and you'll get a pretty good replica.
With MIB you also have the fact that several companies have bought the license to make replicas over the years. These companies were sometimes given the originals to reverse engineer or to take specs from or to make castings. Although there was never any mass produced of Kay's pistol there were some made as prototypes.
The original MIB gear is still held at Sony Studios and was just briefly shown on SYFYs Treasure Hunters, 2nd episode, I believe. You may have a hero version but I would doubt it is the only one out there, especially since there was 2 movies.