You can get the vitamin/probiotic at Tractor Supply for $13.99 60gm, a local farm supply store, or order it online. Amazon has it for 7.99 plus shipping here http://www.amazon.com/Poultry-Provita-60-30-days/dp/B00B1O80GA.
So sorry to hear that you have had a fatality, it is so...
My wyandotte chicks were doing this also. I started supplementing their water with Poultry Provita which is vitamins and probiotics. They no longer shake their head randomly and are all doing well at 15 weeks old. Enjoy your chicks...they grow up sooo fast!!