You can buy KNO3 here: http://www.aquariumfertilizer.com/
Are you dosing ferts?
Chrona is right though, you should find out "why" your water looks that way. Have you tested it? Is it looking that way straight from the tap?
Regular filter may not be enough. You can "polish" your water by using a diatom filter, like a Magnum 350 or the HOB or the Vortex models.