The term "pre-junior" varies by breed, but they are young rabbits that do not yet meet the qualifications for show. If I remember right, you're seeking Holland Lops, which have a minimum weight of 2 lbs. So pre-juniors would weigh under 2 lbs.
I do not, under any circumstance, recommend buying Holland Lops younger than 4-6 months of age if your goal is breeding and showing (which I think I remember from another post). Holland Lops are very, very competitive and go through an "ugly" stage from about weaning to 4-6 months old. When you buy Hollands that young, it's very difficult to know what you're getting. You may get lucky, but you also risk ending up with rabbits that do not mature to be show or breeding quality.
As Hollands reach about 6 months old, you can get a better idea of how they'll mature.