There's actually a more-science-than- you-want-article on this, Paul, in a recent "Aikido Today." The hardest, believe it or not, is osage wood, which is extremely common in the Midwest known as "osage orange" or "hedge" trees. Grows very straight and is used for fence posts by farmers. However, don't know where you could buy it.
What's unique about osage is that it's very hard, but also has a good "spring" resilience to it.
Someone makes them. I saw one two weeks ago at ASU's Summer Camp in Washington, D.C. I didn't get a chance to ask the aikidoka where he had gotten it. I suppose I could track him down, since I think he trains at Aikido Shobukan Dojo.