I haven't bought any mice online yet, but I asked the question about which was the "best" source (in another thread). Several stated that they preferred The Mouse Factory, saying they have the cleanest mice, and that they come vacuum packed. They are more expensive than Rodent Pro. Some people had issues with Rodent Pro mice. Like I said, I haven't purchased from any of these and so don't have first hand knowledge.
I understand the shipping charges are pretty high, though, so I'm not sure just how much cheaper it is than Arctic Mice when you consider all the costs. I'd be interested in hearing what others have to say.