Ordering baby chicks, is it harmful?

[debid]

I think my first step would be to arrange for a necroscopy to diagnose the disease. This way you aren't guessing at what you're dealing with.
If you have a university with an ag department in your area, it may even be pretty inexpensive.

 

[lazy gardener]

All good advice from these fine members. Do you have a roo? If you do, you might consider hatching from your own flock. As pointed out by PP, IF you have Marek's dz. in your flock, you must have some birds who are resistant to that particular strain. If I was dealing with Marek's dz. in my flock, this is the path I would take. While the vaccine can prevent death from Marek's, it will not prevent the bird from contracting it or becoming a carrier. A very real risk of the vaccine is that it is "leaky". Check out this news clip before making a decision.

https://www.pbs.org/newshour/science/tthis-chicken-vaccine-makes-virus-dangerous

In addition to breeding my own replacement birds if Marek's was present in my flock, I'd encourage wild turkeys to visit my property. They carry a less lethal strain of Marek's. If no wild turkeys in the area (I am blessed with flocks of turkeys that wander through) I would try to import some used litter from a local barnyard flock of turkeys, in hopes of bringing in a strain of turkey Marek's.

You could even seek out (chicken) hatching eggs from a local farmer who has turkeys in his yard.

 

[Folly's place]

If the OP has enough birds, and roosters, to hatch a lot of chicks, and is willing to watch the failures (sick and dying birds) in order to produce enough survivors to go on with, that is a plan. I hope to never have to face that!
'Breeding for resistance' is definitely a way forward, but it does depend on having the numbers available to still have an egg producing flock of survivors.
Starting with outside chicks, unvaccinated and with no actual inherited immunity, will be very difficult.
Mary