Where will you be getting your birds? A hatchery, feed store, or other? If you have a specific hatchery we can tell you which to order if we know the hatchery. If from a feed store, well good luck. Each feed store is different to a certain extent, even if they are the same national chain. Some feed store people are more knowledgeable than other, but if you talk to them they might know what you are talking about. They should have a bin labelled “pullets” but whether these are sex links or not, who knows. They might, they might not.
Sex links are made by selectively breeding certain colors and patterns. Because of dominant/recessive genes if the parents are set up correctly genetically you can identify the sex of chicks at hatch. With red sex links the male chicks have yellow down and the females are red. With black sex links the males have a spot on their head while the hens have solid black down on their heads.
I use that rolling replacement system Egghead is talking about. Production per hen varies by individual hen, by their age, and by time of year. On the scale we operate on you will not be able to maintain a steady supply of eggs, it varies too much for different reasons. At peak lay you will get a lot of eggs, but at the off season production can drop a lot. At peak lay 7 hens can possibly give you more than 3 dozen eggs a week, mine do, but other times if they are all the same age you might drop real close to zero, or maybe all the way to zero. It’s one of those trial and error things, try something and see how you need to adjust. That’s where replacement pullets help.