If you have the same power in both eyes and have no other vision problems, you can choose your black reading glasses over the counter which offer reading glasses with much lower price. However, if you have different power in each eye or you have astigmatism, you'd better go to the optical store to get prescription reading glasses that is customized to you. Once you get your prescription, you can order black reading glasses with your prescription online. There are so many online optical stores offering cheap reading glasses and you can search them on your own.
If you search online, you can find that so many sites that you can get cheap black reading glasses. Besides, there are some sites even offer free glasses. I just have got a pair for one of my friends at Firmoo.com. And the glasses frames are very affordable at that sites. Besides, other glasses shop sites like 39dollarglasses.com, glassesusa.com or glassesshop.com/ also provide cheap black reading glasses.