I don't think you need an eye exam appointment at Walmart. Walmart have optical departments in which you can get eye exam and eyeglasses or contact lenses. You can just go your local Walmart to have a look. If you really want to make an eye exam appointment, you can visit https://www.walmart.com/cp/Appointments-at-the-Vision-Center/538522. Hope it helps.
"If you want to make sure that the staff there have enough time to give you an eye exam when you visit, you can call them in advance and arrange the time. You need to find a Walmart store near you and call them. Here is the link: https://www.walmart.com/cservice/ca_storefinder.gsp?serviceName=VISION_CENTER#detailedSearch Sometimes you don't even need an appointment. its just a grocery store and you can just walk in and get an eye exam. "