I've had decent luck with chicks for eggs from Orscheln (or however you spell it!) and there is no minimum. They usually get different breeds in weekly or biweekly and are sexed. However, a pen of pullets means that there is something like a 98% chance of them all being pullets--I ended up with a trio of white rocks from the pullet pen.
If you get sex-linked chicks, such as golden comets (great layers, gentle personality), they will be pullets.
Good luck!