Stuff sacks are a standard backpacker's item, but the coated, waterproof ones are most used by kayakers and canoeists, where gear might get wet. In Death Valley it won't be a matter of getting wet so much as getting dusty. Try REI, Campmore, Bass Outlets or any big outdoor retailer for stuff sacks. They come in all sizes, but get small ones to wrap lenses in, medium size for film holders and a bigger one to store your focussing cloth. I also find that an apron with pockets, or a tool belt or vest with many pockets comes in handy where you can't set anything down.