i seem to have rotten luck finding good stock on cories and every time i bring some home, there are the inevitable losses. but the ones that remain really endure.
Just a quick plug: Try Redkahuna on here. I got my Corydoras from him- granted I've only had them two weeks- but all 12 are healthy and happy... he tank raises them all- so they're more likely to do well in the aquarium than others you don't know the history of.
I got mine from him because he had so many rave reviews on his sale thread from others... that and it only takes 4 or 5 before shipping is paid for because his prices are so low. Glad I did after hearing that people have trouble keeping store-bought ones alive.