I have seen corn free ones, but I think they were also organic (i.e. $$$). It's a bigger movement than you'd think, since Micahel Polin's book came out.
I know two separate farmers who grind their own chicken feed, simply so they can market "soy free" eggs and broiler meat. Some people with allergies apparently cannot have it otherwise.
The mechanism by which soy (or corn for that matter) becomes part of the chicken which is then past on to the human causing a reaction, I could not begin to explain, though.