Ammonium hydroxide is not something that you can isolate the way you can do for ammonium chloride or some other compounds.. NH4OH is NH3 dissolved in water.
NH3 + H2O-->NH4OH
NH4OH was conceived by Savnte Arrhenius as a way to make NH3 to conform his definition of what a base was. There is no NH4OH in solution when you dissolve NH3 in water. It readily forms NH3.
You cannot buy pure NH4OH in the same way as you buy NH3 or pure H2O