You can also mottle your own fabric. Take a spray bottle and put regular bleash in it. Lay a piece of fabric in your bathtub, (or outside on a piece of plastic if it's big), and spray with the bleach. You can have it in a mist setting or more of a stream, your choice. Check fabric after 30 - 45 seconds, if happy with the look, rinse in cold water, you can then wash it if desired, and then dry.
Some fabrics you will need to let the bleach set for longer, just keep checking. Black will not become gray, but orange.