Tylan50 is really hard to find right now. Ordering Tylosin would be your best bet. "Tylan" is the "name brand" of Tylosin. https://www.jedds.com/shop/tylan-soluble-100-g/
I wish I had some, I would send it to you :(
Good advice from @Eggcessive
NC also have 4 state labs - Raleigh, Monroe, Elkin and Fletcher/Arden, you might want to contact them to see if they can do some testing to find the cause. Ask them if they can accept swabs to test for Mycoplasma, Infectious Bronchitis and ILT...