I have been getting fish online/shipped in for years. It depends what fish as to where I would suggest one go. I do not order from any of the big online retailers who also sell fish- ie TPP, FosterSmith or Petsolutions. I mostly buy from smaller size operations. Some are breeders, some are importers and some do both.
What fish are you looking for? I will make suggestions based on that
I've had really good luck from both pet solutions and live aquaria. Unless you are looking for wild-caught or F1, or are planning on setting up breeding stock, I don't see anything wrong with using a larger reputable seller.
If you are looking for bichirs, the absolute BEST person to get in touch with is Toyin at Rehoboth Aquatics. Check out his website: www.rehobothaquatics.com
That's where I've gotten my 2 rare bichirs from: teugelsi and palmas palmas.
I used to buy anubias and a few other plants from Toyen. I have spoken with him on the phone as well a couple of times. I can recommend him as a seller for sure. However, I have not purchased from him in a number of years .