The Pompeii Antiquarium is located near the Porta Marina entrance, the main entrance to the ruins. Opened in 1861, the original antiquarium once housed the first plaster casts made in Pompeii. Although many other Pompeiian treasures were exhibited here, the plaster casts of the bodies were perhaps the largest draw. Unfortunately, the antiquarium was destroyed by Allied air raids in 1943. It was rebuilt after the war and reopened in 1948. Eventually, it was closed again in 1978 for restoration. Time (and funding) moved slowly...and over thirty years later, the restoration is still in progress (according to latest accounts), but it is worth checking to see if it has opened. See photos below.
(If you want to read about the trials and tribulations of Pompeii, here is a translated article from Corriere della Serra, a popular Italian newspaper from Milan. (Thank you, Rick Bauer for the information.)
According to a guidebook from the 1950s,
In the passage are exhibited some of the gruesome casts of human bodies. They have been taken by the simple and ingenious method of pouring liquid plaster in the hollows that had been left in the layers of ash. Death by asphyxia has stiffened these human shaped as they were when they pantingly breathed their last, so as dramatically to display to the onlooker's eyes the last moments of the tragedy with which the life of the buried city ended forever.
For more information about the plaster casts of Pompeii, read