Kingfisher is indeed the place to go for info. ph. 802 295 9908
The article James G. did for ATM was fantastic. They shold offer reprints. One of the main factors to consider is the proper relationship of the different factors. A weapon should be hard, have flexibility, and be stable. the hard weapons tend to be a bit brittle. I would not spend a lot of money on a beautiful hardwood bokken that you are going to use in hard practice. Purple heart is a good material for heavy bokken but I think in a jo it is a bit too brittle. The white oak weapons from kiyota company are real workhorses and they offer the most extensive set of designs apart from the custom makers. but the white oak is not stable over time. As the vibration from repeated impact effects the cell structure it breaks down resulting in a spot that you can't sand down to solid wood. For most of you this isn't much of a problem as this takes a long time to occur for the average user. But I run through a bokken each year when I am using white oak.
I am currently enamored with Kingfisher's "Impact Grade Hickory" It has fine hardness (a bit lower than some of the tropical hardwoods)but has great resilience and is very stable. According to the tests Kingfisher ran on the different materials this was one of the most stable of the woods. I have been giving mine a workover and it keeps on cooking. Definitely more stable than the white oak but a bit pricier. You can save money on these if you don't insist on getting wood that is uniform in color (I prefer wood grain myself).
Hope this helps.