Source: Wikipedia

You ask "Why would I want to buy tumbleweed online? In fact, why would I even want to buy tumbleweed??" You would if you are a fan of Hollywood Westerns, to use them as props in TV and movie productions, as an alternative Christmas tree or for research. I am not kidding. According to this online site, Nasa buys them to test their Mars tumbleweed rover. The business owner, a woman in Kansas, makes $40,000 annually selling tumbleweed online. Since these plants occurs naturally, she incurs no expenses too. Just a little effort to gather them.
Check out the site: http://www.prairietumbleweedfarm.com/

In fact her tumbleweed has such a following in Japan, there is Japanese translation on the site!