Hufflefluff
Songster
I'm going to go against the grain here and say just get some more chicks from the feed store. Is there a slight risk of them endangering your chicks? Yes. But it's very slight, and there will always be a risk when introducing new chicks, no matter who or where you get them from. Mareks isn't rampant among hatchery chicks- odds are overwhelmingly that they'll be fine. And a vaccination doesn't always mean they won't contract Mareks, just like not having one doesn't mean they will. I totally commend you for wanting the best for your chick, but she will be so much happier with a friend or two her age, to the point where it is absolutely worth the risk. And you should ask the feed store where they get their chicks/ if they're vaccinated- after all, they very well could be. Best of luck to you and your chick 
(edited for spelling errors- oops!)

(edited for spelling errors- oops!)