My 11yo son is extremely hard on glasses and we placed an order with Zenni Optical a few months ago for 4 pairs of glasses.
The glasses are awesome, the low price is unbelievable and so far he hasn't broke a single pair (which amazes me!). I will never again buy my glasses from the optical shops. You can get glasses for as low as $8 per pair, plus $4.95 shipping for your entire order, no matter how many you order. Most of the glasses run in the $12-$25 range.
The only downside...they are made in China and it takes about 4 weeks to get them. I have read some stories of them getting hung up in customs and it can take a couple of months to get them. So if you are in a hurry it's probably not the best route.
I found them due to a recommendation from The Clark Howard Show. He ordered several pair and had them tested by his eye doctor. They passed his eye doctor's quality test and were the exact prescription prescribed.
You do have to do a measurement of the distance between your pupils. But it is extremely easy to do and they explain that on their website. The rest of the information you get from your prescription. If you don't have it, call your eye doctor and get a copy.
Although there are different sizes, there really are 3 sizes, women, men and children's. They may vary by a few mm, but nothing really drastic. If you look at your current glasses they contain numbers. Those refer to the bridge width, length from front of glasses to end of sides that go behind your ears and something else I can't remember!

So that will give you an approximate idea of what size you need.
We did have one pair of the ones that we ordered that my son lost the nose pad and screw off of. We took it to
Walmart and they put new ones on and charged us $2.50. His fit fine, so no fitting needed. But you can pretty much take them to Walmart or any other optical shop and they will bend and fit them for you if needed.
