I don't think there is any way to be sure that you are buying females at that age unless the seller is willing to send samples to a lab to determine sex for you.
If you are trying to sex them by sight, the most important thing to look at is the comb. For a female, it should be small and pretty much flat to the head, and not too wide. Males will have larger, more pronounced combs. If wattles are present, that's an indication of a possible male too.
If it is really important to you to only get females, you should buy older birds. They will cost more but with an honest seller you will then get exactly what you want.