I've tried quite a few online places and have been generally satisfied with what i've received.
Saltwaterfish.com sent very nice liverstock. They have a minimum order of 79 dollars but I don't generally have any problem meeting that. I bought a flame angel from them for my boyfriend and it arrived in beautiful shape, very healthy. He tried 2 others from the local places here and both died in not to short a time. The one from saltwaterfish is still alive and well after 5 months and cost about half of what they're asking for them here now.
I love sealifeflorida.com. Ken is easy to deal with, he always sends nice stuff and if I have a problem with anything he's always willing to work with me. (Problems are not the norm) Everytime I've placed an order he has always throw in some surprise freebie. He's tossed in some cerith snails I think it was 6. On 2 occasions he tossed in some free zoos and they weren't just plain old brown ones either, in fact both times they were an interesting blue or mix and 1 other time he tossed in an extra gorgonian to boot. I've only ordered inverts and gorgonians from him so far so i don't know what the fish are like from him. the mushrooms I've gotten have been nice.
I tried corals from coralplus.com on 1 occasion and he sends nice stuff too, decent size, no complaints really I would order from him again.
Marineflora.com has some nice zoos, i ordered a small zoo pack and they were all colorful etc.. some were small though and 1 did arrive dead and when I emailed him about it I never even received a response, i'm not sure if that's the norm for them i've never tried it again.
My suggestion if you're going to order on line, make the order worth the shipping charges and watch the descriptions. Make sure you read what they are sending for sizes so you don't find yourself disappointed. If they don't list it, email them and ask, i've found they're all pretty good about getting back to you with the info relatively quickly.
I find that I like online ordering, you generally get a much better variety to choose from especially if you're like me and don't have a lot of local choices. I was concerned at first about what i would get and what kind of condition they would arrive in but in general i've been pretty pleased.
(sorry for the length, hope it helped)