Quite some of us had used Ace Ammonia Janitorial Strength Formula from Ace Hardware. However, the bottle is huge and you do not need that much ammonia if you only plan to cycle one tank.
There is another commercial available product called Ammonium Chloride Solution by Dr Tim's Aquatics. Some people have had good experience with it. It is actually mentioned in the article you have read.
By the way, do you have a liquid test kit? Get the API Freshwater Master kit if you haven't. It is one of the mandatory supplies if you want to do fishless cycling.
If you only plan to get 6 corydoras catfish, and 12 Danios, it will be fine. Could you please take a closer look at that Fluval filter and give the exactly model? It is important to have a filter system rated high enough to handle all these fish.
