When buying cases online, or anything for that matter, remember to order only from websites and organizations that you trust.
I like doing my cases and accessories shopping on amazon and on eBay. My experience with these websites are mostly good and I know that I can trust them with my credit card information or with my Paypal.
Amazon has a great and ever growing variety of cases for galaxy S4 with photos and description of the products. You can also read reviews by other people who bought this case and see what they have to say before you decide making the purchase.
More Information: Best Samsung Galaxy S4 Cases