When (if ever) does United release business * Alliance availability?

[SPGCont]

I have heard great things about Air New Zealand's/Singapore's/Swiss business/first class.

I am trying to find availability using United miles but NEVER see business/first class seats available.

Whenever I see something that looks worthwhile when I select it I get a message "due to fast changing partner availability...this
flight is no longer available"...

Is it even possible to get such seats?

Has anyone here ever flown Air New Zealand or Singapore Airlines in business/first using United miles? If yes, HOW?

[MAJ]

flying to air new zealand this coming sunday from hong kong to london. booked through aeroplan