For the last 10+ years Ive ordered my fish exclusively from reputable online vendors. But, different vendors for different things. Like my discus, for instance, I order from 2 different vendors who specialize in discus. My South American cichlids I order from a vendor that specializes in cichlids. And so on... I have 6 different online vendors that I order from. I rarely have any problem with ordering from these vendors because they are reputable- good at what they do from customer service to packing and shipping fish.
Also, and this is important, buy only from vendors that offer overnight delivery. You will pay more but you will get a better quality fish from the get-go. Recently I bought a pair of dark rams on aqua-bid and didnt pay attention to how they were going to be shipped... The breeder sent them, to me USPS and they didnt get to me for 8 days. So stressful for fish--- most would die from this, but these ones ( fortunately) did not.
I started buying online because the LFS that was wonderful-- open for 40+ years- closed 10 years ago. The sad thing is, no one has moved in to match the quality of product and service that they provided. I do find this online, however.