Most are made by Aquila in Mexico and are sold by a number of sources:
http://www.midwayusa.com/product/162...lets-box-of-20
These were originally developed for a weird looking pump shotgun that was imported for a short time.
It had a magazine like a pistol in the butt, so it needed short shells.
As I recall the thing operated the opposite of a standard pump gun. You pushed the fore arm forward, then pulled it back to load.