I got myself a pair of prescription sunglasses earlier this year from goggles4u.co.uk (yeah, I know, stupid name) for £20 including shipping. They sent me the wrong color frame, but they did offer to replace them for free because of it, and they gave me a 20% off discount coupon for the next pair I order. I didn't care much about the color, because I just wanted some bigger sunglasses to take on a trip. I had an old prescription from before I left the US which may be a little bit off but my eyes have hardly changed since I was 13 so they were fine for my purposes.
I measured my own PD by taking a felt-tip pen (not a permanent marker!!), looking in the mirror with my current glasses on, and marking a little dot where each pupil is. I then measured the distance. Not super-accurate but probably close enough, especially if you only want a spare pair or some goofy trendy sunglasses, like me.