SuperC,
There is no such thing as "Nitric Acid Crystals", so I am not surprised they are hard to find. Nitric acid (aqua fortis in the old latin) is HNO3 and is only commonly available as a liquid of various concentrations and purities. As PPatch correctly points out, what you are looking for is the nitrated iron salt (Ferric Nitrate) made by dissolving iron in nitric acid and then desiccating (removing all the water and driving off any residual acid resulting is solid crystals). It is hard to get what you want from a chemical supply house unless you are asking for the correct item. Little differences (like ferrous vs. ferric) make a big difference in ordering chemicals.
And, although "aqua fortis" (i.e. just nitric acid) can be used as a stain alone, on this site "aqua fortis" is often incorrectly used to refer to nitric acid with dissolved iron in it. Not the same thing at all.
