What color Rose Comb Dorking are you looking for?
The only Rose Comb Dorkings that are from a hatchery (that I know of) are from Sandhill Preservation Center, and they don't vent sex their chicks, so there's no way for you to know if you got a little pullet or little cockerel chick. You could reach an agreement with a friend or neighbor. They could order the chicks, raise them up until you could tell which ones are pullets vs cockerels, and then you could buy a pullet from them. If you can find a breeder local to you, you might be able to buy a pullet, once they're old enough to tell the boys from the girls. I don't know of any breeders that vent sex their chicks. So if you only want one, and it has to be a female, and it has to be a Rose Comb, you're pretty much stuck with buying them when they're old enough to tell the difference in gender. Very few breeders or hatcheries will ship the larger birds, and the cost of shipping is very high, so you'll need local.
Do you need a Rose Combed bird? If you could take a Single Combed Silver Grey Dorking, you could order from Murray McMurray hatchery. They vent sex their chicks, with at least a 90% guarantee of accuracy. You would need an order of 25 total chicks, but they don't all have to be the same breed. You could split an order with another person(s), and just ask them to mark the Dorking pullet. (You really shouldn't raise just one chick at a time -- they're social animals, and need company. Two is an absolute minimum, but 4-5 is best to avoid stress.) Or you could find a feed store that is willing to special order the Dorking pullet(s) for you when they order from their hatchery. A friend of mine asked her local feed store to add in a few Dorking pullets and one cockerel to their regular chick shipment, and they were glad to accommodate her. They ordered from Ideal hatchery, which don't have the best Dorkings in the world (MO), but she was quite happy with them. They were sweet birds, laid a reasonable number of eggs, and were wonderful pets. She wasn't breeding them, so she didn't care if they were close to the SOP or not.