I've also heard good things about the Wetspot. Shipping is expensive but the fish are supposed to be of high quality. I'm eventually going to order an Anostomus Anostomus from them along with some other fish. As mentioned, they be have a huge selection of rainbowfish.
I like Divers Den on liveaquaria as you get to see the actual fish you are buying and they go through a minumum two week quarantine which includes a dip for parasites. DD and regular liveaquaria fish are guaranteed for two weeks. The others I'll mention only guarantee live arrival. This is why send of their prices can be high: they have to factor in the guarantee into their prices.
Some of the liveaquaria prices are okay, especially when you hit the $99 threshold for free overnight be delivery. They usually have 2.5-3" clown loaches in sets of three for $69.99 on Divers Den.
I've ordered from Imperial Tropicals. I've ordered millennium albino and regular rainbowfish. The males have a really deep color that's amazing. I would order from them again. They have a $9.99 two day shipping with live arrival guaranteed deal (it's something like a $45 min purchase).
Full disclosure, I have a red millennium male that somehow had a messed up eye. I'm not sure if it be happened during shipping and I don't notice or it got damaged when I transferred him to the QT tank. His color is great and I don't want to blame them for what happened. (I have a thread about what treatments I used). It's not contagious as none of the QT fish ever developed anything. I think its just permanently damaged but he eats and spawns and has great color so I believe their product is good.
All three of these places have good reviews, which is why tried two of them and be will order from Wetspot.