It is easy task, you can just search it in goggle to see which sites offer large round glasses. Of course, you shall read some reviews of the site which you tend to buy large round glasses from so that you know if the site provide good quality glasses. Besides, you can also search some large round glasses at eBay or Amazon.com if you needn't prescription.
Well, that's no big problem. You can buy big round glasses everywhere without efforts. But what matters is that the price can vary greatly. If you choose to buy them in shopping malls, it's expensive, but they're also cheaper online or in some small shops. I recommend Amazon.com, where you can find out selective ones.