I got mine from a concrete creations business near the house, they use the bags as padding for statues, etc.
Feed store might have some available, or (this is the way I might go next time) piece goods (fabric) store. It's really cheap by the yard, and there is a selection of tans and darker browns to choose from. I thought about mixing up the colors in blocks and stripes next year. Lasts about a year, unless it's constantly under water. Does help to wet it when you have a hose around, though, it ages the appearance.
Moss Mix
1 Qt buttermilk
1 big tub plain yogurt
lots of different kinds of moss, crumbled
Mix well with an electric mixer and paint it on burlap, rocks, bricks, etc. Good project for the kids, can't really screw it up.