You can have a look at https://www.polyvore.com/round_sunglasses/shop?query=round+sunglasses There are many kinds of round sunglasses. But I have never placed an order in this store. So I don't know the quality of their products. If you want to buy one of them, you'd better have a good look. Good luck!
There are some round sunglasses on ebay. https://www.ebay.com/bhp/round-sunglasses I think they are similar with John Lennon's sunglasses. But to be frank with that, I don't think round sunglasses look better than ray ban sunglasses. So I prefer to ray ban sunglasses. But this doesn't matter. You can choose what you like. You have the right to do that. Hope this helped!