Live Aquaria is the most reputable but don't be surprised to find that over half their fish are "out of stock" and they supposedly email you when they are available (I've never got an email) but they still carry a more diverse selection than your local fish store. Shipping charges are reasonable.
Aquarium Fish.net - also carries a diverse number of fish and if you spend more than $169 your shipping is free. They must not have had what I' was looking for because I've only ordered once from them.
Imperial Tropicals is one of my favorites - I've placed 5 orders in the last couple of years. I don't recall what their shipping costs are but I believe they are resonable.
Arizona Tropical Gardens - I've ordered from many times because they nearly always have whatever obscure fish I'm looking for. Prices are good but you will PAY PAY PAY for overnight shipping. The others mainly charge about $49 for overnight shipping which usually includes a styrofoam box, all the fish double bagged with a good amount of air and heating pads or cooling pads depending on the weather where you are. I've never had one die. But they always make a mistake on their smaller orders (they generally sell to aquariums, fish stores and even NASA. My last order which I placed due to the number of unique Gourami as well as Glass fish I couldn't find anywhere and a Betta for an empty but cycled 5 gallon tank and then sent me 2 identical Betta's - so I had to rush out and purchase a tank, heater- the filter and lights came with it. Now these Betta's are stunning BUT they charged me $79 to ship, $9.99 for the Styrofoam box and $1.99 for a cold pack - and get this - three weeks after arrival a number of the new fish came down with ICK which I've never, ever had a problem with - so I'm treating it now. I will never order from this store again. They always make a mistake in the order but it's the only online fish store where I've picked up a disease. I guess I need to re-set up a quarantine tank after all - but I don't know if I would ever quarantine an entire month. They have awesome selection of fish but are just not careful. They breed most of their fish on site.