We've had great success using a site called Zenni Optical. When sailing, it wouldn't be unknown for one of us to make a spectacular "donation" to Neptune, so we like to have a spare pair at all times.
Even with all the add-ons, like varifocals, transitions lenses, and the coatings, they're a fraction of the price of the High Street, and they come pretty quickly too. Of course, you have to know things like your pupiliart distance and so on, but most of the numbers you need you can garner from your existing frames, and the help guides.