[Frogger]

It is not illegal in Canada only controlled. Mix it with a little diesel fuel and you got yourself a pretty powerful bomb (Oklahoma Bomber)

I got mine from a local fresh water fish supply company. They use it in fresh water to grow plants.

You will need to sign your life away and I believe can only purchase a pound at a time. Although a pound of KNO3 will last you a long time.

If you want really pure KNO3 you can purchase is from a scientific supply company like Fisher Scientific (>99% pure). The amount of KNO3 you will be adding to your tank I am not sure you need the purest form.