It is possible to buy canisters of laughing gas online if you hold a license to purchase medicines – because nitrous oxide is controlled as a medicine unless it is sold for culinary purposes.
If you want to buy nitrous oxide online read this about Laughing Gas
In some countries then is is legal to purchase nitrous oxide but only for specific purposes and inhalation is not one of those purposes!
It was only a few years ago that laughing gas was sold openly in night clubs read this for the story. It would seem that the law was broken by the people selling and not the people using the drug. It is also interesting to note that selling it for recreational uses even with a licence is considered to be breaking the law. I guess this is to stop dentists turning their surgeries into the modern equivalent of the Victorian laughing gas parlour parties.
The interpretation of the laws regarding recreational usage of laughing gas on private land eg. at festivals seems to be very flexible. Observational evidence would suggest that two blind eyes are averting their gaze from the potential misuse of Nitrous Oxide in public by large numbers of people. It would certainly be easy enough to find the culprits – after all balloons are difficult to hide!