Kubota doesn't make them. Your dealer needs to order them from someone like ATI, Woods, WR Long, FFC, etc. I am sure I forgot some of the manufacturers, but as you can see there are several. Many of the sponsoring dealers on this site sell them as well. Carvers sells the WR Long, ATI is also a sponsor as well as others.
It does not seem uncommon for many dealers not to be familiar with them. I know my local one wasn't familiar but was willing to look into it.
I ended up making my own ordering the teeth (made by FFC) and having a friend weld it together.
Good luck,