Yes, Maracyn 2 is very easy to find. Do you live near a PetSmart or other large petstore with fish supplies? Not sure about Wal-Mart though. Not sure what meds you have but its always good to have some basics on hand. If you post what you have, we can suggest anything you may want to consider buying.

I wouldn't add more fish to the tank right now until you are sure Dopey is healthy and any meds you have put in the tank have been effectively removed.

Do you have a QT? Any fish you get should be Qt'd for at least a few weeks. As for getting a betta.. Yes, you have to be careful who you put a betta with. I don't have tetras but if they are at all "nippy" I wouldn't advise a betta because they will go after the bettas fins.