Buy online!
If you normally go to buy your glasses at the brick and mortar store you will end up spending more than you have to.
If you can follow simple directions as give them, the online store your prescription and your facial measurements so your glasses will fit right. They have videos on how to do it then you can get really cheap prescription glasses online.
So if you already have a up to date prescription just go here if your looking to order a cheap second pair of glasses.
http://www.zennioptical.com/ if the link dont work just copy and paste it into your browser.
Good luck!