I am trying to perform the demonstration of a strongly endothermic reaction reacting barium hydroxide with ammonium nitrate. I have been scouring the web for ways to get barium hydroxide, but none have showed any promise. The only way I have seen to obtain it is by heating the nitrate to decomposition and reacting the oxide with water. I would rather avoid this as I have no way to deal with the nitrogen dioxide gas that would result.
You can buy barium hydroxide. See the listing from Flinn Scientific although Flinn is by no means the only seller. However, Flinn's safety information is written at a very accessible level.