I just use sand from my Ace hardware "RMC Pacific Lapis Lustre". It says right on the bag for sandblasting. It is filtered so there is a very consistant grain size throughout.
Of course, if you pour the used sand back from the bag into the gun a few times, in a short time the sand will get pulverized into baby powder like consistency.
I don't know if you saw in the link but the "Hot spot" has a number of rubber tips for matching the shape of the metal you are spot blasting, it works real well to contain most every grain.