Answer

OK, thanks for your reply. Based on multiple dogs coughing suddenly, this is most likely to be an infectious, contagious problem, that the one dog carried home from the veterinary clinic. Kennel Cough is very contagious through airborne transmission, and because some dogs seem to have a greater immunity, they can act as a carrier and spread it to other dogs, even if they themselves have no symptoms at all.

Unfortunately, antihistamines such as Benadryl are not likely to be helpful in this situation - you will need antibiotics and possibly cough suppressants, or the problem can progress deeper into the respiratory tract and result in a much more serious issue. I understand that money is tight, but unfortunately there isn't any other option, since you need to obtain a prescription for antibiotics. You could ask for a written prescription, and have it filled at a discount human pharmacy, which may result in some savings.