Source of KNO3, K2SO4?

Hello everyone,

I'm working on getting my algae under control, but have been
unable to find a source of KNO3 and K2SO4. I've heard that plant
fertilizers have these chemicals, but often contain other things that
make fishkeepers nervous. I checked the local science supply store but
they told me they couldn't order them. I'm a bit worried that lab-grade
chemicals might be too expensive anyhow (ie, ScienceLab.com wants $77
for it's cheapest K2SO4!)

Are there some easy sources for these two chemicals?

--
Ross Vandegrift


"The good Christian should beware of mathematicians, and all those who
make empty prophecies. The danger already exists that the mathematicians
have made a covenant with the devil to darken the spirit and to confine
man in the bonds of Hell."
--St. Augustine, De Genesi ad Litteram, Book II, xviii, 37