Since my chicks were about 2 weeks old I've been scooping sand/gravel out of my stream a couple times a week. I spread it on a cookie sheet and bake it in my oven at 400F for 20 or 30 minutes. I figure that will kill any germs in it, and it dries it out nicely. Once it's cool I just put the whole cookie sheet right in the brooder. They mostly scratch it off the pan, but seem to peck up the right size grains of sand. Someone else mentioned digging up a hunk of sod and putting it in the brooder. I did this once, it was fun, but messy. Mostly the ground is still too wet around here.