I know there are a few koryu which prefer straight bokuto despite their use of curved katana. Not really sure why. At any rate, there is at least one available at http://www.swordstore.com
I think I remember reading that the shinkendo folks tend to break a lot of bokuto so they tend to prefer the straight ones, because they are easier to manufacture. Don't crucify me if I've got this wrong. It's been quite a while since I recall reading this.