It may be more time-effective to just go to the docks and buy it.
As for mining, there is a post in the crafter's forum asking about it. One of the devs stated that each first attempt to dig in an 8x8 resource grid has a chance to pop a niter deposit. As such, you would theoretically have the best luck just hitting once in a spot, then moving to the next spot in the grid until you hit a deposit. Each spot's chance to spawn a niter deposit resets every 5 min, I believe.