Poor Chewie :(
Aquabid has pretty fish but shipping them here is out of the question, so almost all my bettas are from walmart and a few from petco. Even at a petstore, most employees are clueless when it comes to bettas.
If you and your boy want to give it another try, here is my guide for picking a walmart or Petco fish:
1) Look for one that is Alert, not necessarly active (they are in a tiny cup, so they probably won;t be too active). If they respond to you picking up their cup, that's good. And sometimes when you look at them in their cup, they respond by giving "the look". I can't really explain "the look" but when one gives it to you, you will know
2) avoid bettas with rapid gill/mouth movement. I'm not really sure what causes it, probably ammonia poisioning, but gasping for air and rapid gill movement is not a good thing.
3) Check their body. avoid ones with white salt like dots (ick) and white fuzzies (columnaris or fungus).
4) clamped fins - most likely from bad water conditions in their cup but it can also be a sympthom of something more serious.
5) Pineconning - if looked at from the top, the bettas scales stick out like a pine cone. It can be a sign of Dropsy which is usually fatal.
6) Ripped or missing fins - probably fin rot which is easy to cure via aquarium salt (NOT table salt). However sometimes it is a sympthom of something more serious.
Well, that's about it for my fish buying guide.
Even if you had maracyn 2, there was still a high probability that the fish would of died. Columnaris usually kills quickly but there is a strain of it that kills more slowly.
As greentea said, it is good to have certain meds on hand, just in case. Maracyn and Maracyn 2, if you can find it. I had to order mine off ebay. Also aquarium salt and plain old epsom salt are also good to have on hand.