So, I searched for one of these over the past few years. And since my own thread would pop up each time I did a search, I thought I'd update.
The antique versions are near impossible to find locally. A few have shown up on ebay, but proved to be too expensive and heavy shipping.
This year we are going to use a commercial french fry cutter, like the one pictured. We are going with more sweet potatoes then mangels, since they are more adapted to our area. The commercial fry cutters say they are 'not recommend' for sweet potatoes. but we don't plan on cutting as much as a restaurant would. Hopefully, they will hold up :0)
The nifty part of these cutters is that they can be wall mounted, the potatoes then fall right into the feeding bucket.
When I feed them to anything other than chickens, I just whack them with a machete. Chop them into 2-4 chunks. I've never had a problem with anything being able to eat them. For chickens I mount them whole on a stake of rebar and let them peck away at them.