where to get wyvern mkII besides from cache?

Some USS or Triada stations have them; can't really remember which one.
Definitely needs FTU warehouse access though.

The thing with a station's inventory in Star Wolves is that if you can't find certain items there, you can just leave and then re-dock for the game to randomize the items being sold.

That said, it seems that every station has its own 'inventory set', and will only randomize items from that set.
So if a station's inventory set doesn't include a Wyvern Mk. II then it'd not show up for trade no matter how much you re-dock at the station.

You can get one in one of the quest lines for finishing a fraction mission.