You can just go to the store where you buy the sunglasses. Usually that kind of eyeglasses may send you a eyeglass case for free when you buy it. If you need one, you can just buy one from there. Or else, if you don't want to go around, you can just buy it online which may be at the cheap price.
Where you get the big sunglasses, where you can get big cases. If there is not what you want, go to a big optical shop. You can find there are varies kinds of cases. Some can offer customized service. Otherwise, log on ebay or other online shop without regional limits, there must be a style meet your requirement.