Not sure about Deutschland : ) but in the US I picked mine up from a hardware store, it's typically in the cleaning supply section, janitorial strength but its not 100% ammonia. Mine is the standard something like 10% I believe just make sure it doesn't have any dies or added fragrances.
How long has your tank been cycling?
It sounds like you are on your way. The ammo starts to drop, you get trites, trites drop and trates rise and the next day your trites will be gone and the trates will shoot up.