The majority of what I buy is from the LFS (ABC reefs and ABC reefs 2) or at frag swaps from fellow reefers. However, when I do buy online, I usually buy from liveaquaria, and recently put in an order with pacificeastaquaculture. I've also bought from bluezooaquatics.
Liveaquaria is great. I've only had two corals that didn't make it, and one they actually didn't even send, but sent me a note along with the rest of my order, saying that the coral started to look bad, so they didn't want to send it, and had already refunded my money. It put my order below their free shipping amount, but didn't charge me for shipping. In the other case, the coral started to die the day after it was put in my tank, so I called liveaquaria. They told me to keep them updated. When it died two days later, I gave them a call, and they refunded me my money without even asking for pictures. I do recommend buying corals and fish from them from their Diver's den, because they are quarantined better, and the sizes of the corals are bigger than if you'd just order out of the regular sections.
I haven't had any problems with bluezooaquatics. They do send an acclimating kit along with any order, which is really nice.
I most recently made a purchase from pacificeastaquaculture. My order got here REALLY early in the morning, which was awesome. Everything was packaged really well, and the corals have been doing great so far in my tank. They actually called me later on in the day to see if I received my shipment, and to see if I was satisfied so far with my purchases and the packaging. They then told me to give them a call if I had any issues at all, and thanked me again for my order. I thought that was pretty cool.
As others have said, I'd try first to get corals or fish from one of the local fish stores. I'd definitely recommend that with fish. But if you decide to order online, those three places, in my experience at least, are decent places to order from.
