Where can one purchase a tampon case?

It depends on where is more convenient for you - pharmacies and supermarkets for commercial tampons, or health stores for organic tampons.

Many women are also using online services, either buying tampons in bulk from web sites such as Amazon or using services like The Period Store or Be Prepared Period that allow you to set-up a monthly delivery of sanitary products - these services can save money, are more convenient, and best of all give you a wider veriety.