You do not need a prescription for these medications. Try looking at a local farm supply store (like
Tractor Supply). You should be able to find either Tylan or Gallimycin (or something similar to Gallimycin, like Terramycin or Duramycin).
Also, what makes you think that it is CRD? What are the bird's symptoms? Many respiratory diseases look similar, so you may be dealing with something different. Antibiotics will treat bacterial diseases like CRD, but won't work for viruses. So, if you've mis-dagnosed, you may not see improvement even if using antibiotics.
As far as I know, CRD and other respiratory diseases can affect all types of poultry, including Guineas. However, I'm not sure on this.
If your flock has CRD (or another respiratory disease--most respiratory diseases cause chickens to be carriers)and you introduce new birds, the new birds will likely get the disease too. They might not show symptoms, but if the other birds are carriers, I think that they will become carriers too.
Keep your birds' immune systems strong by giving some electrolytes and probiotics. See if you can give them some nutritious foods, too, like scrambled egg, yogurt, and applesauce. Just remember to not give dairy products while giving antibiotics.
Hope this helps! Sorry your flock is sick!
