not that i'm aware of. Go down any local pvc companies or even local tip and have a look if any of the old doors have something that would work. I tend to keep them from old door locks we throw away as this part has a habit of coming unscrewed and either falling off or jamming the door! Looking at your pic it looks like a ROTO lock and i know i don't have any of those
Just about to say its a Roto, never seen latches sold separately though, as Ronnie suggest maybe a trip down to some local DG companies and have a rifle through their skip