Call your local veterinarian tomorrow morning and ask how to get in touch with your areas diagnostic lab! It usually is connected with a University and the local Extension service can also tell you- and looking it up tonight online.
But, you really need to not ask us, but ask someone that can test, find out what is what and go from there. It could be a Vite deficiency to a toxin or even a disease and you do need to know this before adding more to your flock, it could be something in your soil where you may have had birds before, or something you just brought in on these guys. Like many of us say, its not that you need to be worried, fretting or beware, but its bringing in facts so you know to BE AWARE.
The stress that birds go through in transit and free range so soon after bringing in, it is nervous to them. Keep them in a pen for a few days until they are fully aware of where to return to for safe roost, food and water. (Sounds like they are fine, but they need adjusting time too)
Good luck!
(edited for this) I was confused by the new ones you bought, I thought they were these that died. But you do need to have a diagosis if you have one more die. Find out how to "store" the corpse until the time comes to deliver it. Also, some of these tests can be done by blood and saliva, so don't think you have to have another dead one or euthenize one for the testing)
