You would need to ask at your local Tractor Supply if they are vaccinated, but since they have to pay extra to get them vaccinated it is highly unlikely they are. The chicks at my local TSC are not. I do not vaccinate the chicks I hatch.
The vaccine is actually Turkey Marek's. It does not prevent the chickens from getting Chicken Marek's, it prevents the growth of the lesions that cause the damage. It's possible for vaccinated chickens to catch Marek's and infect other chickens. The vaccine does not cause them to be infected and will not cause them to be carriers. They'd have to catch it from somewhere else to be infected.
For the vaccine to take effect they need to be isolated from chickens that do have Marek's for two or three weeks. If you brood them indoors you can achieve that but not if you raise them with the flock.
If you think about it, people that hatch chicks with broody hens can't effectively vaccinate for Marek's. If it is in the flock they are immediately exposed. Some that hatch by incubator do vaccinate, many don't. Very few chicks you get at a feed store are vaccinated. Unless you pay extra when you order them from a hatchery they won't be vaccinated. Marek's is a very nasty disease once it gets into your flock, but many of us have non-vaccinated flocks and don't have a problem.
If you know you have Marek's in your flock then vaccination is a great tool to help manage that. Some people don't want to take that chance and get them vaccinated. Some of us don't vaccinate without issues but if it does show up it can be pretty horrible. It's a personal decision.
