Is readily available ammonia a substitute for ammonia water?
Ammonia at standard temperature and pressure is a colourless gas. So when your recipe says "powdered ammonia" there must be some kind of mistake. It might refer to a salt, that can be produced with ammonia and hence serve as a source of it. One of such salts is ammonium chloride.   

In household stores available ammonia is a solution of the gas in water. Depending on the product there might be other ingredients as well, for example surfactants if used as a cleaner. In any case there should be some information about the concentration of the solution. Often given in percent and it strength may vary, but I doubt it contains more than 10% in any case.

In conclusion, the available ammonia is ammonia water.

Lastly a word of warning. As an artist you are probably handling paint thinners and other chemicals all the time. Treat ammonia just as carefully as these. And keep your place well ventilated. You will probably do this anyway, because the smell is just horrible.