I've gotten healthy fish at several places. However, I've gotten unhealthy fish from the same places.
Dallas North and Fish Gallery have advanced life support systems and seem to take good care of their tanks. Boutique keeps smaller systems and puts the date of arrival on the outside of their tanks so you know how long the fish have been there.
Remember, the fish don't originate from these stores, they are bought from importers (mostly in California) and have been through some rough capture and shipping procedures before they get to Dallas.