I buy my fish from Seahorse, Wet Pets, and Elmers. If you look for healthy specimens who have been in their tanks for a while, I've found that any of those three stores have had 100% survival rates for me. Ich happens in shipping for a lot of fish (especially tangs) and if you look closely, you'll see it in any pet store...although big box stores like Petco seem to salt their fish, and you can tell that it's not just a small outbreak due to stress of shipping. A lot of the larger tangs that Seahorse and Elmers have that have lateral line were returns from customers, not actual products of the conditions in those particular stores, so overall I think it's a bit unfair to judge the quality of their systems based on a few individual fish. That's not to say that if I see an outbreak where every fish in the tank has ich, that I don't steer clear on that particular visit.
I never had any luck at all with liveaquaria...not sure why. NYAquatic is the only online vendor that I have, or would use due to an incredibly good experience with them, and the policy they have that they won't ship a fish that isn't eating in their tanks. Had a regal angel fish (generally considered hard to keep) shipped to me and was healthy and eating the first day. They have very good pricing too.
Overall however, I won't buy fish online, just because I like to see my exact specimen in the tank, and like to know that he's been healthy and eating for at least a week in the store's tank before I take them home. Any retailer should be willing to feed their fish for you to show you that they are eating. You run a much better chance of longevity and survival if you are patient and picky.
