Princess auto carries this, they call it an art die. They carry a reasonable selection of dies now.
My roller is from them as well, just the usual cheap 18" thing like harbour freight used to sell.
You have to make them more rigid so the two arms don't walk back and forth under load or you wont get nice results. I also strongly recommend replacing the crank with a steering wheel, unless you have a 7+ foot arm span. With the wheel, motorizing no longer seems like the priority I initially thought it was.
Lots of info online on modifying these.