Depending on the pressure available and the type of horn, manual or motorized valve. If available pressure is higher, you could go down on diameter and actual bore of valve. If source is above 2 bar and below 10 bar, you should be able to find solenoid valves with a varity of current/voltage solenoids within your specifications,and actual boring could possibly go down to, say 1 mm (??? you will have to test).
Check for instance Norgren, SMC, and number of others.