Tubs are generally sold in 6" increments, rough-opening measured to the studs (not the finished walls), so it is highly unlikely that you have a 64" surround, it is almost certainly a 66" one. These are usually part of the tub, and not an add-on, so often are not available separately. Now, you could have something like a solid-surface one custom made out of Corian, Silestone, etc. any size you want. If your tub model is still being made (unlikely), you might be able to buy a direct replacement for it, but parts costs may be as much as a whole new tub. The alternative would be to possibly tile the area, but it may be less expensive to just replace the tub with a new one and its matching surround.