Steering boxes can be found on some of the japanese type pick-ups (car-size) or smaller 4 wheel drives, or another possibility might be something off a ride- on lawn mower
The trouble with go carts and rideon mowers is that the steering is too direct. I want 2-3 turns lock to lock, they are usually less than 1 turn lock to lock - especially go carts.
I saw a vitara steering box last night, looks ideal, I think that's what I'll use.
Cortina Mk1/2, Anglia, Pop, most late 60's motors will still be on a box, plenty common, the cortina ones especially, lots of people convert to rack and pinion.