That's why I have found it is healthy to practice good quarantining techniques for all new additions for at least 30days (sucks I know but pays off) before it ever enters the main tank. That way it gives the new fish time to adjust to you, no stress & to not have to compete for food during this time. But also gives you time to take a look for any sign of disease that might show up and then you can treat as needed before an outbreak occurs. It also insures the safety of your tank mates in your main tank resulting in healthier, happier, long lived fish.
Did I also forget to mention it saves you from future stress too? Nothing more stressful IMO, then a tank full of your favourite fish that are hanging on for their life from adding a new addition that infected everyone.
So if you practice good quarantining techniques, who cares where you buy it from. As long as you make smart choices of choosing one thats healthy, make sure its eating (Ask the LFS to show you) and has no torn fins, doesn't breath lethargic, is not pale in color, doesn't lay on bottom of tank its in or shows any sign of disease, than I say.....
GO FOR IT!
Happy Reefing!