You don't need "pure" ammonia, the regular household sauce will do just fine. (The variations in concentration is why the dosage is "enough to get to 5ppm" instead of "x per gallon").
The important thing is that it be straight ammonia and water, no detergents no lemony scents. If it suds up when shaken its no good. There will be a little bubbling, but it settles down right away, much like shaking up a bottle of water. No soapy style suds. Soap kills fish.
Look for the cheapest plainest store brand stuff in your local supermarket. That's where I found mine.