Their eggs are white so they would have to use barred rock roosters. From what I can tell, at least in our area, they use commercial brown leghorns roosters and white leghorn hens.
It might be different in other parts of the country though.
Would it be possible for the offspring to look barred rock completely? Reason I'm asking a friend has two that were given to them. They say are from trader joes and they look very much barred rock from the pictures.