It happens to a LOT of people, & brings much frustration and heartbreak since many chicken diseases leave them carriers for life. Meaning even if the chickens appear to have regained their health, they will still spread the disease to any new chicks or chickens you bring in.
Pullets nearing point of lay or older would be best since you have one older hen already, but if you go that route, be very careful and know that your source has a healthy, disease-free flock. Aquiring chickens from an unknown source is always very risky. Whether respiratory diseases or viruses such as Mareks, you will find many sad stories on these forums from people who unknowingly bought sick or disease-carrying chickens.
The safest route would be to buy day-old chicks from a feedstore or large hatchery, raise them yourself, then them introduce to your hen when they are feathered out. Safest, but probably not the most convenient. Very sad for your loss. I hope you find comfort in knowing you did everything you could.