I've used goggles4u.com and coastalcontacts.com
Goggles4u has some dirt cheap stuff, I got 3 pairs of glasses (1 regular, 2 sunglasses) for about $35 shipped. Not super amazing quality, but certainly good enough. And they are practically disposable at that price. So no need to be worried about breaking your $400 glasses all the time.
Coastalcontacts is usually more expensive, but their customer service really is top notch. Got glasses for my wife (progressive ones with transition lenses) for $150 or so. They developed a "fog" (not sure whose fault it was), but called them up, and within a couple minutes they processed the order for a new one and gave a prepaid label to ship back the old ones as soon as the new ones arrived.
So if you have a fairly simple prescription or don't want frills (transitions, etc) then I'd go for the dirt cheap ones (like goggles4u) and if something happens you can always get another one for dirt cheap. If you want something fancier then I'd recommend coastalcontacts as they will take care of you if something goes wrong.