Saltpeter works too, basically any of the KNO3 or NaNO3 things work for this. ^^
And yep, Saltpeter Stuff is most commonly in the Desert especially with Railcraft installed.
Dynamite is better than Boomsticks for this by the way, while Boomsticks can be made more cheaply using normal Gunpowder from Creepers.
(yes i know Boomstick is technically the word for shotgun, but hey, a stick of gunpowder fits that description too!)