I just bought (today in fact) a 50# bag of KNO3 fertilizer at a Pottery supply place. They use KNO3 with other things to make colors for glazing pottery items.
My local bulk fertilizer place (which I frequent often to buy fertilizer by the ton) does not carry KNO3. If it did it would probably be in the 30 dollar a bag range would be my guess for this time of the year. Fertilizer prices have been all over the place in the last 3 years though, so I am only guessing.
The pottery place charged $50 for a 50# bag, which might be a little high if I could get it elsewhere, but damn cheap compared to grants stump remover at about 6 bucks a pound. Of course, you can make a lot of slow match with one pound of KNO3.
I do know a small, low key pyro supply dude who carries KNO3 for $18 for a 15# bag, plus shipping. I have only bought from him once, and there didn't seem to be any hazardous shipping fees associated with a KNO3 purchase, so it was regular UPS rates. He is in "New England", so if you are in the northeast shipping would be cheaper than if you are in California.
He seems to have a well respected reputation as being a fast, honest shipper and his price list has prices for stuff that are probably cheaper than just about anyone else I have seen.
So, if you can't find KNO3 locally, PM me and I'll give you contact info. He doesn't exactly hide or anything, but he posts on pyro sites that are sort of restricted to weed out the 15 year old M-80 makers and such. Because of that I won't post his email address here.