I've had good service from Goggles4u for single vision specs (decent quality, yet cheap enough to be disposable, albeit they're probably assembled by Pakistani seven year olds), but I think I'd be a little wary of buying varifocals online, at any rate my first pair.
AIUI, there are a lot more ways to get the fit wrong than with single vision lenses, and an optician standing in front of you is more likely to be able to work them out than one who's at the other end of the internet. (Of course, you should feel free to weight this opinion according to the number of varifocals I've bought or chosen, which is zero.)