Just buy saltpetre because you have to age the urine
http://en.wikipedia.org/wiki/Potassium_nitrateHow to use Tinder Fungus (Amadou)
Preparation
Here's the traditional method for preparing Tinder Fungus for flint and steel.
Take a large iron pot, and almost fill it with the fungus. Add strong urine to cover the fungus. Boil gently for three days and nights, adding urine as necessary. Drain and dry.
A more convenient method is to boil the fungus in a solution of saltpeter. Try to get Potassium Nitrate rather than Sodium Nitrate. The Sodium Nitrate is hydroscopic, and the tinder fungus prepared with it works poorly in damp weather.
Saltpetre impregnated fungus - Polyphorus fomentarius - known all over Europe as 'amadoo' - was widely used as tinder from early times. It was collected from old trees and treated by boiling, soaking in saltpetre, then dried and used in a tinder box. This contained flint, steel and material - the tinder - to ignite easily and was an essential household article before the invention of matches as well as a personal possession as the cigarette lighter is today.
You could probably also make char-fungus
out of it,
or at least one piece, which you would crush and smear on a piece of dried fungus to catch spark easier , and you could also add some lead to extend burn time, like a slow-match ...
