These can be purchased through a lab suppliers or simply made from a glass ‘u’ tube and some rubber hose. Mount the tube securely so it cannot fall over and attach a length of Bunsen-type rubber tubing to one of the open ends. Attach a small glass funnel to the end of the tube and stretch a balloon over the wide mouth. Pour some coloured water into the tube and press the balloon diaphragm until the liquid is at an even height in both sides of the tube.
By pressing the balloon, you are creating a difference in pressure between the two ends forcing the liquid to move as it is difficult to compress. When both ends are open to the air, atmospheric pressure acts equally and the liquid level will even out.
If one end of the tube is closed, atmospheric pressure changes will make the liquid move accordingly and therefore can therefore be demonstrated as a crude barometer.
Digital manometers are increasingly being used in schools and some can be connected to dataloggers to accurately measure deviations in atmospheric pressure.