Don't give up! Try again, however LFS have been known to sell unwell fish. If it was a goldfish that died you'd go back to the store and get a new one. I am lucky to have a breeder here in town that has some hearty stock and the fish don't go through some of the horrible shipping that most fish do but my first try with these babies have been a huge success. The 2 I have bought from LFS over the past couple months have been sick. One died and 1 i am still treating<<< looks good however but still...Im so glad i quarantined.
There are some things you can look for at the store when buying discus at the LFS. look at both gills, are they working well. Is the fish playful at the store and eat there? Clamped fins and dark colour. I'm sure you know most this as you have been reading a lot so my comments are hopefully to encourage you not to give up. Its so rewarding to have these beautiful guys grow before your eyes. I've never had a fish that was as social and liked petting etc. So much fun!!!
How big was the fish that died? Are you 100% sure the tank was cycled? how often did/do you do a water change? What are you feeding them? Maybe post a new thread in the sickness/disease section and some experts can chime in to help the little guy thats still in your care.
I'm a Discus newbie but I encourage you to keep trying. I'm not convinced these guys are that difficult to house. I just think it really stinks when you pay a great deal and they die.So I feel for you. You may not feel so frustrated if it was a goldfish.
Try ordering from Hans directly or another sponsor here with great reviews. Or find a reputable place/breeder within driving distance.
I hope you try again and let us know how your future Discus keeping goes!!! Kelly