Black Diamond is only sold at tractor supply as far as I know. Wasn't able to find one near you. Petco sells Black sand it's almost the same thing. If you don't need a lot of it People were selling it on the forum for pretty much just the cost of the flat rate shipping box.
You can check automotive body shops or any place that does media blasting. They may have some or be able to order you some. Home depot sells the blasting machines so they should have some sort of abrasive in store. Lowe's doesn't seem to have it even online anymore.