It really depends where you are, and if there are any 'real' aquarium stores around you. The two I suggested are pretty good, but a lot of sellers offer the same fish as the chain stores. I would never buy any fish online if I could go to a store and see it. Both Angelfins and Maison du Poisson often have things good local stores don't.
You can check for diseases and parasites, see if they look healthy, and see their size at a store. Online, you have to trust the person who packs, whose name you don't know and whose face you'll never see.
Don't even look at the US stores. The border is slow, the paperwork is slower (and sometimes has extra charges) and the tariffs are real. You don't want fish sitting in a depot - the faster you get them, the better they survive.
