The Trencher with Destroyer LMG is considered the Sniper.
Thought as much, but wasn´t completely sure...
For the sniper I would propose to convert it from a Reg Trencher or a model from another game. There are sopm good examples around.
On ebay.com there is a RAMS out, but thats for about $ 100 and you wouldn´t want that.
The AirCav is pretty hard to come by, but perhaps someone has a spare.
And for the ordnance officer... I think that one was only to get in one of the buildings-expansions.
Maybe the Bidge?
Can anyone confirm or contrdict me.
If you want it badly you should be able to get one of these expansion at PA´s still.