I know that @azygous has a flock positive for leukosis that she has dealt with for awhile. RAL does offer testing where you collect samples and send in to them. You could think about switching to another type of poultry for eggs. I suppose.
Sorry for your loss. I would suggest first getting a necropsy by the state vet on a bird with symptoms that is culled. That way you would know what you are dealing with. Mareks is spread is the dust and dander of infected birds. If Mareks is confirmed, then I would not buy new chicks. The...