I got a pair from a well known chain of Opticians. The standard cycling glasses get a pair of clip in lenses to your prescription behind the wrap around of the glasses. I paid around £450 for mine, which got me the frames, three wrap arounds, one clear, one amber and one smoked, the clip in insert and the prescription lenses. As my normal glasses cost me £400-£600 (I have a very complex prescription!) I felt they were a bargain.
Do I still use them? Well no! My prescription changed and buying new inserts at the time was too expensive (My normal glasses frames were £60, the lenses £425 and lenses for the inserts would have run me another £425 or so!) and by then I'd sussed that I steamed up in the wrap arounds on every hill and at every set of traffic lights, removing the glasses meant not being able to see clearly, so I went back to normal glasses on the bike as they don't steam up. But are a bit draughty at 40mph+ down hill!