I have a few fabrics that look like sand. You can usually find things like that in the areas that a store has fabrics that resemble things like brick, wood, grass, pebbles... If they have a section like that.
If you can't find anything, perhaps you could coffee stain an ecru-ish batik. Get a stiff paint brush, shake off most of the coffee into a pail, then hold the brush in one hand, and tap it down on your other hand to spray the fabric. If you don't shake off most of the coffee, the spots will be bigger. Then Iron the fabric with a floursack cloth over the top so you don't smear it. Not sure how else to set it.