Nitrous Oxide (N2O) - Nitrous Oxide, also known as Dinitrogen Monoxide, NOx, or laughing gas, has a boiling point of -89.5 degrees Celsius at 1 atm, and is normally maintained as a liquid at a pressure of 54 bar (783 psi). Nitrous Oxide has a molecular weight of 44.0 and a density of 1222 kg / m3 at 20 degrees. The critical pressure and temperature of Nitrous Oxide is 7.27 MPa and 36.6 degrees Celsius.
The nitrous oxide bulbs for kitchen seem to be 8 or 16 grams of N2O but I can't find anywhere that gives the pressure. They are very cheap.
Bigger bottles can be bought from car performance shops, but not sure if any in Cambridge.
Found on the internet. Might be quite helpful later. http://edge.rit.edu/content/P07106/public/Nox.pdf