Have ordered fish twice in the last year, once from a well-known online firm (TriMar), once from a breeder (also very well-known). Very pleased in both cases, fish arrived healthy and extremely well packaged, and I would trust either of these sources a lot more than I would trust any of my local shops.
I have however taken note of a post of Neale Monks' where he points out that he would rather not buy certain fish online, as you need to check that e.g. a plec is eating and has a full tummy. Having said that, seeing that I have no transport and do not greatly trust my local shops (besides, they don't carry exotic plecs), I might still have to resort to online ordering in a case like that. With the small fish I am after, it is probably not an issue: they are either going to be eating or they're going to be dead.