Is that one of the now defunct Saf-T-Lock company's products? That might explain why you can't purchase it any longer.
I only found one seller of AR-15 magazine locks via a quick Google search -
Marstar in Canada. Their magazine lock doesn't look exactly the same as what you pictured, but it is close:
Marstar does sell to Americans, so give them a call if you are really interested. But do it quickly, since Marstar is clearing out their magazine locks as well.
Is your heart set on a magazine lock? A cable lock, while not as attractive, is certainly more readily available.