Found an easy way to get contraband - simply hop into an anti-aircraft gun and start a bit of mayhem.

Sooner than later Zeppelins will come for you so simply pick one off then turn the gun anti-clockwise and there should be another one to shoot down.

Repeat as there should be another 2 waiting for you.

Now if you turn anti-clockwise again the 1st Zeppelin you shot down reappears - so just keep on shooting them down until you either get bored or get the amount of contraband you require.

I have tried this in numerous locations (from tall buildings work best) and seems to work.