You could try rxsport.co.uk, I have just placed an order with them for some sample Oakley frames to try on and if these are comfortable and fit well I will almost certainly order some prescription sunnies from them. The home trial costs £4.95 and lets you try up to 5 different frames, if you then place an order this fee is taken off the cost of your glasses They stock lots of different manufacturers (Rudy Project, Bolle, Nike, Adidas etc) and you can customise the lens/frame colours for some of these if you want. I e-mailed with some questions and got very rapid and helpful replies and they were nice on the phone too.
Possibly also worth trying Optilabs, they have received very positive reviews elsehwere on RW forums and will also send you samples to try on. I tried this and although they were very helpful, unfortunately I did not find the glasses I chose very comfortable but this could just be my freakish big head!
Edited: 04/08/2010 at 22:21