One of my chickens just died and I don't know why

We just purchased 6 chickens all of different breeds on May 2nd at 3 months of age. I went out this morning and saw my silver laced wyandotte, laying by herself under a tree in the outside part of thier coop. She looked sad and I made a mental note to go out after I had gotten my kids down for thier nap, to look at her a little closer. I returned to find her dead in the same spot where I had seen her this morning. Thinking back now, last night I thought it a little unusual that she was perched by herself. They usually huddle up together. This is really the only thing that I have noticed though. She just seemed sad and was a bit of a loner. I had to have found her right after she had passed since she was not stiff at all. I gave her a once over, as much as know to. Her vent looked totally normal, no lumps to be found, throat felt good, feathers were not fluffed up, eyes looked normal.

I have removed and burried her, but am now scared that I will loose the rest of my flock. I am brand new at this so I'm feeling a little uneducated. I did a little research and the only thing that seems to stand out is Marek's Disease. Where do I go from here? Is this something that I did? Or did she bring something with her from the farm she came from? I'm truly open to any suggestions. Thanks so much!

 

Is this expensive and can I get these things at a local feed store or do I have to have them shipped?

Nichole