Also, I know that many of the bokkens you can get from Japan seem too short - are these OK length?
If this is a problem you can try
Bugei Trading Co
. I haven't gotten one of their hickory bokken yet but I probably will at some point in the future.
Also, if you're thinking of making some jo yourself I've found a supplier of hardwood dowels in various diameters/lengths. For $150 you can get 100, 48" long 7/8" diameter hickory dowels.
This company actually produces wooden weapons used by the Society for Creative Anachronism folks.
When I was with the SCA (Middle Kingdom) we were forbidden from using wooden weapons at any official SCA practice or event (which was all of them). The only acceptable material at the time was rattan. The reason was that when rattan finally fails it doesn't splinter. It gets mushy and then starts to get "hairy". No worry about taking a sliver through your visor. Now they allow swords to be made from something called Siloflex, and also rattan siloflex combinations.