The one that most scuba divers use is Boots decongestant.
It has the advantage that it dosn't have any pain killers or other stuff mixed in with it. It has wads of caffine though. Dosn't make you sleepy and lasts 3-4 hours.
Its in pill form and at 4 quid for 24 is alot cheaper than others.
As for it being legal to use on duty i presume not for flight crew but ATC who knows.
MJ