There are two dealers that I prefer because they have always been friendly and a comfortable relationship has developed. But they simply don't always have what I'm looking for, or haven't been able to locate it in Japan (which is not saying I have a phenomenal collection or anything
). So I buy from multiple dealers. Some as a last resort. On one reputable dealer's fish I have now received ich once, costia once, and bacterial gill disease once, all of which I've taken care of in quarantine. I've never mentioned it to the dealer, and I still shop there.
Do I recommend shopping from multiple dealers?--Not at all. However, illnesses are a part of the hobby. If you owned 20 dogs, you'd be at the vet a couple times every year; so it makes dollars and sense to try and become your own vet. You need a microscope and to be comfortable anesthetizing and scraping--it's just part of the deal. Quarantine is a must. Injections are do-able. Centrifuges are cheap and drawing blood isn't so hard either. And it can be fun building the confidence to handle health issues. Even if you stick with one dealer, placing complete trust in their protocols can be a big mistake. Be prepared.
Now, getting the proper value for your money, that's a whole separate thing.