You are right harrycatcat,there is usually quite a few on e-bay.
It's also worth ringing round the gun shops for any 2nd hand ones they may have in.
I recently upgraded my cabinet to one with a ammo box on it. The local dealer, took my old one in part exchange. Less bother than advertising it.
http://www.thepigeonshooter.com/
paul hart is the man you want.