One has to be very careful where you obtain corys that are being shipped, as JD has just noted. It is not unusual for stores to receive entire bags full of dead corys.
Local stores or suppliers are my personal favourite, because you can see the fish, and if they have been in the store for a few weeks, many of the initial problems with corys will have passed. I will absolutely never buy corys that have just arrived in a store, as they are much more likely to die in your home aquarium during the first few days; give them at least one week, preferably two, in the store so they can deal with the dead fish, not you.
The US stores JD mentioned have good reputations [I've not used them as I live in Canada and cross-border shipping has its own difficulties], and I would add another supplier. There used to be a place called Corys-R-Us and they had a website, and I read good things about them; I believe they were in Florida, but for some reason I can't find the site so they may be gone.
Petsmart was mentioned, but I will never buy fish from them, after a terrible experience that introduced some bacterial issue even with the fish in quarantine. These chains may be better in some areas, but I would not risk it. They obtain their fish from the same source, obviously, so infected fish will be rampant across the country.
Byron.