You can get some at any grocery store, like Safeway. Just make sure it has no detergents or scents. If unsure, give it a shake before adding to the tank. If it bubbles or foams, you do not want to use it. At Safeway, there is a brand, it's made in Canada. It's called Clear Ammonia. That's what I used. It says on the bottle that there's no detergents or scents.
As for dosing, try a capful, wait 10-15 minutes, then test. If it's over 1-2ppm, then you know you need to use a little less, if not quite 1-2ppm, then a little more. If it's a bigger tank like my 55G, I think I used 3 or 4 capfuls. But just go a capful at a time til you get to the level you want.
You want the initial level at 4-5ppm. Once ammonia is starting to drop, then 1-2ppm til cycled is fine.
55G Filstar XP3 - 16" Fire Eel
75G Medium Planted - Filstar XP3 Low Light - Established Feb 2006