You need to go to the opticians to have an eye exam so that you can get the right lense, otherwise they will damage your eyesight when using the computer. Most the optical stores will give you free eye test when you purchase a pair of glasses from them. Don't worry about the price, they will be reasonable.
You can buy some quality computer glasses online in low prices. Now, there are many online optical shop offer computer glasses such as framesdirect.com, firmoo.com and lenscrafters.com etc. As far as i know, you can also expected to get free computer glasses at Firmoo.com. Again, the eyeglasses on firmoo and framesdirect are sold in low prices.