Hello
I think I read somewhere that the Hornby International coupling had been discontinued, but there is a company called Veit that make a Scharfenberg style coupling. Saw them in Germany early last year. They seem to be made for HOe scale trains, but this is 1:87 so would be fine in OO. They seem to represent the mechanical part of the coupling without the electrical gubbins that hangs below, but I'm not sure barrier wagons have this anyway.
Can be found online in packs of 1 pair or 5. Here for example: http://www.modellbahnshop.com/index.php?cat=c12976_Scharfenbergkupplungen.html
You might need to find someone who speaks German though. Failing that, the part numbers are there - it might be worth asking Gaugemaster if they could get some in. They seem to be able to get hold of most German items.
Hope this helps.
Rich