The gas or toxic smoke used by Holmes has been called different things, but it doesn’t seem that people should be able to purchase such products other than the small personal defense sizes.
Does anyone know if adults can legally purchase canisters of tear gas?
Pepper spray (not tear gas) is made from a pepper/spice and can be accessed by anyone in the world. As for tear gas, I have no idea.
You can get pretty large pepper spray canisters as part of an anti-burglary device which fills a room with pepper spray when triggered. Shotgun News has adds for it.