[BoomerD]

I prefer to buy fish from one of the few good LFS that I do business with. I like to see how healthy they look, if they are eating, and if they show any signs of disease. I used to do a lot of mail order fish business back when I bred discus & angels, (no internet!) and never had a problem, but usually was dealing with people I knew or had friends who knew them. I recently got involved with a "group buy" from one of the larger on-line fish dealers, and it's been a nightmare. Several people had fish that died, the order was shipped to more than 1 place, (delivery error) and about 1/2 of the order didn't even come in. Might be several weeks till it's all filled. Not a great experience.
I am a firm believer in quarantining ALL fish. I do not advocate copper for all fish, because many fish are highly sensitive to copper. Angels & tangs are among the list. Quarantining will not completely remove any possibility of ich (I just had an outbreak in a tank with NO new fish due to a heater failure!) or other infestation, but it will greatly improve your chances.
Try to find a good reputable LFS in your area. One that knows your set-up & experience, that remembers what fish you have, and might even try to keep you from buying fish that are not compatble with your current set-up/livestock. I drive about 80 miles to do business with one I trust.