Nitrogen Dioxide is a chemical that forms an equilibrium with Dinitrogen Tetroxide, which in the world of rocketry is often just called "Nitrogen Tetroxide" so as to increase the amount of confusion. If you have a tank of one, it will change between being one or the other based on temperature or pressure.
This is one of the commonly used hypergolic storable oxidizers, and, like all commonly used hypergolic storable chemical propellants, it's quite dangerous and probably not suitable for the amateur. Specifically, it's quite deadly if inhaled.
Nitrous oxide, N2O, is much safer and liquefies under pressure similar to propane, and is commercially available due to its use in the whipped cream and motorsports industries. However, it has lousy performance and is not hypergolic with typical fuels. Additionally, it definitely can explode under the right cirucmstances. See this presentation.