When you shop in a store, it is nice because you can pick up and touch the objects you are looking at. You can see how they feel, get a better idea of the colors and also get a better idea about size. This is true for any fabric or piece of clothing you may be interested in. The same is true for your futon covers. You can get a futon cover at your local store for about the same, maybe a little more (less if there is some big sale) than getting one online in most cases.
So, it seems like there is really no reason to shop for your futon slipcover online. Well, not for the reasons mentioned. Those will always be easier to gauge in person than over the computer. However, where you will find shopping online to be of the greatest benefit is in your time savings. It will take you extremely less time to shop for up to hundreds of styles online than rummaging through them in person.