Pete,
It is a very common farming product. Probably some don't know but if you add Ammonium sulfate to a sprayer you can use half as much Glyphosate (Roundup) and still get the same killing effect. Go where your local farmers buy their fertilizer and chemicals and they will have it. It probably will not say the NPK on the bag but it will be 21-0-0 the reason it does not say is most is used with the herbicide as I mentioned. It is cheap..here it is $13 for a 51 pound bag, yes 51 pounds, no clue why the odd amount. At a teaspoon per gallon that will last you a LONG time:) There is a water soluble one which is what you want, it dissolves just like miracle grow IE instantly. Make sure you keep it sealed as it is horribly hygroscopic and will turn in to a brick.....you can rebreak it no harm done but just easier if you keep it dry in the first place.