As recommended elsewhere in this thread, EyeBuyDirect is quite good if you have enough knowledge to specify your own glasses and fitting. They are owned by upstream BigLens™, so lens quality is comparable to many opticians, with low pricing since they make their margin on frames. If you need custom fitting height for one of their frames, you can measure and return after receiving the glasses, then they will remake a new pair for free within 14 days of receiving the first pair.

I've used GlassesUSA.com before as well. I kind of switch between them depending on who has a better sale for my kids' glasses.