Affectionately, we like to call them "Nang Crackers", and they are available over the internet. A friend of mine ordered about 5 (for herself and mates) and showed me hers when it arrived. (We are in Perth, Western Australia)
They are small & made out of metal, shaped a bit like the connector of a garden hose (that plugs the hose to the tap), but with a holster for the charger.
They couldn't possibly be made out of plastic - they have a strong metal pin in the centre for piercing the cream charger! (Just like a cream whipper machine)
Basically you put a balloon over one end, load the charger into the holster, twist and hold tight!!! Cos the initial burst of gas is really fast and really strong and we've watched balloons fly off and waste a whole charger of gas
It takes about 60 to 90 seconds before all the gas empties from the charger and the balloon is ready to breathe from.
I'll try and track down the website that she ordered her "nang cracker" from. We found them to be especially handy to keep in the glovebox of the car!
Although, in line with my given title of Apprentice High Priestess of Nang (or, more lovingly, Trainee Nang Bitch), I like to take my 1 litre silver beast out with me and have even managed to sneak it into a rave inside a large black bag!
Bee safe with your nitrous .. and Never Trip Alone.
Those last three words of wisdom were from my GP
Happy Trails in woma-woma-woma-land!