I was thinking of getting these
http://www.amazon.co.uk/SUNGLASSES-Performance-PRESCRIPTION-Interchangeable-Protection/dp/B005MJ6CXS/ref=sr_1_fkmr1_1?s=sports&ie=UTF8&qid=1358181977&sr=1-1-fkmr1
They are £80 which includes the prescription lenses and 3 different shades which means you can pick the best shade for the weather.
They look like exactly what I have, only a slightly different shape to the earpiece rubber covers. The mechanics are the same, the nosepiece, the fittings for the Rx clip, and the overshades clip fitting, and the overclips are the same colours.
I have clear, shooter's yellow, orange, brown, and polarizing gray clips. The most useful here in Ireland are the yellow for night work and the orange for most days. The polarising clip is carried on the bike but rarely necessary; however, it was only a tenner so I'm not sorry I ordered it.
It might be smart for you to buy a complete spare frame, or spare parts like an extra nosepiece and an extra Rx clip, to ensure the longevity of these glasses, because buying from Amazon puts you even further away from the suppliers and, as I explained above, I already had trouble getting spare parts, thought he local B&S representative when pushed supplied a substitute. Of course, this assumes you want spectacle frames that last. If you're buying fashion items to be discarded after a year, they'll last at least that long.
Ha! An optician that I promptly dropped sneered that I'm just unfashionable and cheap, that smart people buy new frames every year, that I should own different frames for different outfits. I listened, said thank you politely, crossed the street and said to my new optician, "Fashion is what I do to other people. Tradesmen don't do it to me. Are we in accord?" and he said, "I see herself has been giving you her sales patter." I wasn't the only customer who was brassed off by the woman's presumption and brazen greed.
BBB also has an Rx model in their catalogue. It's more expensive and less convenient (the tinted lenses have to be fitted individually). But at least with BBB you have somewhere to go for virtually guaranteed spare parts.
Andre Jute