Once you know what type of glasses you want to buy, you can search retro sunglasses online. There are many online optical stores selling retro glasses because they are also a trend in recent years. And you can compare them to choose the cheapest one to get the retro glasses.
Either you can buy them from online optical stores or you can go to your local second-hand clothing shop where you can find retro glasses. The local second-hand clothing shops prefer to collect the old styles, including the retro glasses. And you can try them on before you buy them so that you can know whether the retro glasses are suitable for you or not.