With the Hauser bomb, he had for the first time used a mixture of aluminium powder and ammonium nitrate, producing a much bigger bang and signaling to Ronay that worse was to come.

http://nymoral.blogspot.com/2008/09/theodore-kaczynski-unabomber-part-five.html

So that's how he did it ---you can make aluminum powder and NH4NO3 is a common ag chemical. He was in Montana.