You can get the sand in any of the Construction sites or in the places where they sell building materials. Just see to that the sand is dried before filling the stands/ speakers or else the chances of Rusting is more for the stands. First insert a Plastic cover inside the stands / speakers and then fill it with sand.
Not sure where you get that SPECIAL sand in India. But I think its a Marketing strategy the companies adopt in selling these type of products. I do not find any reason a normal dry sand will spoil your speakers if you fill it properly.
Agree with Venkat here. You are looking to add weight only. Make sure the sand is sieved and does not contain particles or other stuff. When I set up some bookshelves, I have lined it with construction quality plastic wrapping bag into it. My friend got a tailor to stitch it so it fit in the wood stand neatly before filling in. Make sure you ahve 2 - 3 layers of lining as you dont want to ruin the stands by any chance. If its metal, make sure the one layer of plastic is the toughest one you can find.