How about these:
Cargo Plane (only obtainable in the mission 'Minor Turbulence'
Titan (I know it can always be found at the spawn point but it's almost impossible to obtain)
Yellow Frogger (only spawns in the air, serious exploits are required to obtain)
Annihilator (currently only available online)
Train (only obtainable in the mission 'Derailed'
JB 700 (only obtainable if purchased on the in-game internet, weaponised variation only available in the missions 'Deep Inside' and 'Pack Man'