It's currently available on Acanta CD 44 2089-2.  There is a desirable
companion disk of Orff's shorter Monteverdi realisations, "Ariadne's
Lament" and "Ballo del Ingrate" (both performed like Orfeo in German)
which you might also find appealing.  That is on Acanta CD 2090-2 with
more or less the same excellent forces.

They are both exciting examples of the richness that can result when
one great composer is re-invented by another.  As you say, the remarkable
thing is how like himself Orff makes Monteverdi sound!

Christopher Webber,  Blackheath, London,  UK.
http://www.nashwan.demon.co.uk/zarzuela.htm
"ZARZUELA!"