To success buying eyeglasses online, you can ask your friends for some reliable online glasses shop. If not and you have no idea where to buy prescription glasses, you can search it in Google. I heard that coastalcontacts.com, framesdirect.com and firmoo.com are good sites to buy prescription eyeglasses. Of course, you need judge it yourself so as to get best suitable eyeglasses. You can search some reviews about those sites to help you determine where to go. Good luck.
Almost all optical stores offer prescription glasses. But if you want to order prescription glasses online, you should go to have an eye exam first to get your prescription. Then you can order glasses you like with your prescription. You can just search them online and then there will be a list of online optical stores appearing on the first page, which are all reliable. You can try Zenni Optical, or eyebuydirect.com which have a wide selection of prescription glasses for you to choose.