Nitric acid is fundamental to all nitrations, making it one of the more indispensable tools of the trade. It is also difficult to produce, concentrate, and transport (safely). You can get it online at several suppliers, but most if not all of them require you being affiliated with some institution if you want it over 70% and for a decent price. If you're not, you have to make your own. There are almost
countless methods of producing it. To name a few, xNO3 + H2SO4, xNO3 + Sodium bisulfite, and a few others that I can't remember right now
I just made a thread on an easy way to get low concentration HNO3 to about 95-98, in case anybody wanted to make RDX, or just to get absolutely killer yields on any nitration. BTW, while on the subject of nitrations, adding a little NH4NO3 to nitrating mixtures will help a bit.